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GNDU Question Paper-2024
Bachelor of Computer Application (BCA) (Hons.)
5
th
Semester (Batch 2024-28) (CBGS)
ZOOLOGY: Paper-Zoo-V (A)
(Developmental Biology)
Time Allowed: Three Hours Max. Marks:35
Note: Attempt Five questions in all, selecting at least One question from each section. The
Fifth question may be attempted from any section. All questions carry equal marks.
SECTION-A
1. Write notes on the following:
(a) Egg maturation
(b) Role of subtesticular cells.
2. Explain oogenesis and role of follicles.
SECTION-B
3. Describe gastrulation.
4. Explain embryonic development of Herdmania.
SECTION-C
5. Give an account of development upto three germinal layers and their fate in chick.
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6. What are the steps of metamorphosis in frog ?
SECTION-D
7. How is placenta formed? What are its types and functions ?
8. Explain embryonic development of rabbit.
GNDU Answer Paper-2024
Bachelor of Computer Application (BCA) (Hons.)
5
th
Semester (Batch 2024-28) (CBGS)
ZOOLOGY: Paper-Zoo-V (A)
(Developmental Biology)
Time Allowed: Three Hours Max. Marks:35
Note: Attempt Five questions in all, selecting at least One question from each section. The
Fifth question may be attempted from any section. All questions carry equal marks.
SECTION-A
1. Write notes on the following:
(a) Egg maturation
(b) Role of subtesticular cells.
Ans: 1(a) Egg Maturation (Oogenesis)
Introduction
Egg maturation, also called oogenesis, is the biological process through which a female's
immature reproductive cells develop into a fully mature egg (ovum) capable of being
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fertilized by a sperm. This process is essential for sexual reproduction because only a mature
egg can combine with a sperm to form a new individual.
Think of it like growing a seed into a healthy fruit. A tiny seed cannot produce a new plant
immediatelyit first has to grow and develop. Similarly, an immature egg must undergo
several stages before it becomes capable of supporting the beginning of a new life.
Egg maturation begins before the birth of a female, pauses for many years, resumes at
puberty, and continues every month until menopause. It is a long and carefully controlled
process regulated by hormones.
What is Oogenesis?
Oogenesis is the process of formation, growth, and maturation of female gametes (eggs)
inside the ovaries.
The word is made from:
Ovum = Egg
Genesis = Formation
Therefore, oogenesis means the formation of eggs.
Where Does Egg Maturation Occur?
Egg maturation takes place inside the ovaries, specifically in small sac-like structures called
ovarian follicles.
Each follicle contains:
One immature egg (oocyte)
Supporting follicular cells
Hormones and nutrients required for growth
The follicles protect and nourish the developing egg.
Stages of Egg Maturation
1. Formation of Oogonia (Before Birth)
During fetal development, special cells called oogonia are produced by repeated mitotic
divisions.
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Oogonia are diploid (2n), meaning they contain 46 chromosomes in humans.
Millions of oogonia are formed before birth.
These cells are the starting point of all future eggs.
2. Primary Oocyte Stage
Before birth, oogonia enlarge and become primary oocytes.
Important points:
They begin the first meiotic division.
They stop developing during Prophase I of meiosis.
At birth, every female already has all of her primary oocytes.
These cells remain inactive until puberty.
3. Growth After Puberty
At puberty, hormones stimulate some follicles every month.
Main hormones involved:
FSH (Follicle Stimulating Hormone) stimulates follicle growth.
LH (Luteinizing Hormone) triggers ovulation.
Usually, only one follicle becomes dominant each month.
4. Secondary Oocyte Formation
Just before ovulation:
The primary oocyte completes Meiosis I.
Two unequal cells are produced:
o One large secondary oocyte
o One tiny first polar body
The secondary oocyte receives almost all the cytoplasm because the future embryo needs
nutrients.
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5. Ovulation
During ovulation:
The mature follicle bursts.
The secondary oocyte is released into the fallopian tube.
This usually occurs around the middle of the menstrual cycle.
6. Completion After Fertilization
The secondary oocyte begins Meiosis II but stops at Metaphase II.
It completes meiosis only if a sperm enters.
After fertilization:
Mature ovum forms.
Second polar body forms.
Male and female nuclei fuse to form a zygote.
If fertilization does not occur, the egg degenerates.
Hormonal Control of Egg Maturation
Several hormones work together:
FSH → stimulates follicle growth.
Estrogen → helps egg maturation and prepares the uterus.
LH → causes ovulation.
Progesterone → prepares the uterus for pregnancy.
Without these hormones, normal egg maturation cannot occur.
Importance of Egg Maturation
Egg maturation is important because it:
Produces healthy eggs.
Maintains chromosome number.
Makes fertilization possible.
Supports normal pregnancy.
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Ensures continuation of the species.
Simple Diagram
Oogonium (2n)
│
▼
Primary Oocyte (2n)
(Meiosis I begins and stops)
│
Puberty
│
▼
Completes Meiosis I
│
├────────► First Polar Body
│
▼
Secondary Oocyte (n)
│
Ovulation
│
Fertilization
│
▼
Completes Meiosis II
│
├────────► Second Polar Body
│
▼
Mature Ovum
│
▼
Zygote
Conclusion
Egg maturation is a slow but highly organized process that starts before birth and continues
until menopause. Through oogenesis, immature cells become mature eggs capable of
fertilization. Hormones regulate every stage, ensuring that reproduction occurs normally.
Understanding egg maturation helps us understand female fertility, pregnancy, and human
reproduction.
1(b) Role of Subtesticular Cells (Interstitial/Leydig Cells)
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Introduction
Subtesticular cells, more commonly known as Leydig cells (interstitial cells), are specialized
cells located in the spaces between the seminiferous tubules of the testes.
Although they do not produce sperm directly, they are extremely important because they
produce testosterone, the main male sex hormone.
Imagine a factory where workers make products. The seminiferous tubules are the workers
producing sperm, while the Leydig cells are the power station, supplying the hormone
needed for the factory to function properly.
Without Leydig cells, sperm production and normal male development would be severely
affected.
Location of Leydig Cells
Leydig cells are found:
Between seminiferous tubules
In the connective tissue of the testes
Close to blood vessels for hormone release
What Do Leydig Cells Produce?
Their main product is testosterone.
Testosterone is responsible for:
Male reproductive organ development
Sperm production
Secondary sexual characteristics
Sexual behavior
Production is stimulated by Luteinizing Hormone (LH) from the pituitary gland.
Major Roles of Leydig (Subtesticular) Cells
1. Production of Testosterone
This is their primary function.
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Testosterone:
Develops male reproductive organs.
Supports reproductive health.
Maintains fertility.
2. Helps in Spermatogenesis
Although sperm are formed inside seminiferous tubules, testosterone produced by Leydig
cells is essential for this process.
Without testosterone:
Sperm production decreases.
Fertility becomes poor.
3. Development of Secondary Sexual Characters
During puberty, testosterone causes:
Deep voice
Beard and moustache growth
Growth of body hair
Increased muscle mass
Broad shoulders
Male body structure
These changes make boys develop into adult men.
4. Growth of Male Reproductive Organs
Testosterone promotes normal growth of:
Penis
Testes
Scrotum
Prostate gland
Seminal vesicles
5. Maintains Sexual Function
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Leydig cells help maintain:
Sexual desire (libido)
Normal reproductive function
Healthy hormone levels
6. Maintains Bone and Muscle Strength
Testosterone:
Increases muscle growth.
Improves bone density.
Gives physical strength.
7. Hormonal Balance
Leydig cells work together with:
Pituitary gland
Hypothalamus
This hormonal feedback system keeps testosterone levels balanced.
Hormonal Regulation
The sequence is:
1. Hypothalamus releases GnRH.
2. Pituitary gland releases LH.
3. LH stimulates Leydig cells.
4. Leydig cells produce testosterone.
5. Testosterone supports sperm production and male development.
Importance of Leydig Cells
Leydig cells are essential because they:
Produce testosterone.
Support sperm formation.
Maintain fertility.
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Develop male characteristics.
Keep reproductive organs healthy.
Maintain normal sexual function.
Simple Diagram
Hypothalamus
│
GnRH
│
▼
Pituitary Gland
│
▼
LH Hormone
│
▼
Leydig (Subtesticular) Cells
│
▼
Testosterone
│
├──► Spermatogenesis
├──► Male reproductive organs
├──► Deep voice
├──► Muscle growth
├──► Beard & body hair
└──► Fertility
Conclusion
Subtesticular (Leydig) cells are vital supporting cells of the testes. Their main role is to
produce testosterone, the hormone responsible for male growth, reproduction, and fertility.
They indirectly support sperm production, regulate the development of male reproductive
organs, and bring about the secondary sexual characteristics seen during puberty. Without
healthy Leydig cells, normal male reproductive function and fertility cannot be maintained.
2. Explain oogenesis and role of follicles.
Ans: 2. Explain Oogenesis and the Role of Follicles
Introduction
Oogenesis is the process by which female reproductive cells (ova or eggs) are formed inside
the ovaries. It is one of the most important biological processes because it allows
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reproduction to take place. Unlike males, who produce millions of sperm every day, females
are born with a fixed number of immature egg cells. Throughout their reproductive life, only
a small number of these eggs become mature.
The development of an egg does not happen alone. Each egg is surrounded by a group of
supporting cells called follicles. These follicles protect, nourish, and help the egg mature.
They also produce important female hormones such as estrogen and progesterone.
What is Oogenesis?
Oogenesis is the process of forming a mature ovum (egg cell) from an immature germ cell
inside the ovary.
"Oo" means egg.
"Genesis" means formation or creation.
In simple words, oogenesis is the journey of a tiny immature cell that gradually develops
into a mature egg capable of being fertilized by a sperm.
This process begins before the birth of a baby girl and continues until menopause.
Stages of Oogenesis
1. Multiplication Phase
During the development of a female baby inside her mother's womb, special cells
called oogonia are formed.
These cells divide repeatedly by mitosis, increasing their number.
By birth, no new oogonia are formed.
Simple Example:
Think of planting many seeds in a garden before the rainy season. These seeds are like
oogonia waiting for the right time to grow.
2. Growth Phase
Some oogonia enlarge and become primary oocytes.
The primary oocyte stores nutrients and grows in size.
Each primary oocyte is surrounded by a follicle that protects and nourishes it.
At birth, the primary oocytes remain in a resting stage until puberty.
This stage may last for many years.
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3. Maturation Phase
After puberty, every month hormones stimulate one primary oocyte to continue
development.
The primary oocyte undergoes meiosis-I, producing:
One secondary oocyte (large cell)
One first polar body (small cell that usually degenerates)
The secondary oocyte starts meiosis-II but stops halfway.
It completes meiosis-II only if fertilization occurs.
Finally, it forms:
One mature ovum (egg)
One second polar body
Only one functional egg is produced from each primary oocyte.
Flow Diagram of Oogenesis
Oogonium (2n)
│
Mitosis
│
Primary Oocyte (2n)
│
Meiosis-I
│
┌───────────────┐
│ │
Secondary First
Oocyte (n) Polar Body
│
Meiosis-II
(Completed only after fertilization)
│
┌───────────────┐
│ │
Mature Ovum Second
(n) Polar Body
What are Follicles?
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A follicle is a small sac present inside the ovary that surrounds the developing egg.
It acts like a protective home for the egg.
Each follicle contains:
One developing egg (oocyte)
Supporting cells
Nutrients
Hormone-producing cells
Without follicles, the egg cannot mature properly.
Stages of Follicle Development
1. Primordial Follicle
Present from birth.
Contains a primary oocyte.
Very small and inactive.
2. Primary Follicle
The follicle begins to grow.
Supporting cells multiply.
The egg starts developing.
3. Secondary Follicle
Follicle becomes larger.
Fluid starts collecting inside.
Estrogen production increases.
4. Graafian (Mature) Follicle
Fully developed follicle.
Contains the mature secondary oocyte.
Ready for ovulation.
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5. Corpus Luteum
After ovulation, the empty follicle changes into the corpus luteum.
It secretes:
Progesterone
Small amount of estrogen
These hormones prepare the uterus for pregnancy.
If fertilization does not occur, the corpus luteum degenerates into the corpus albicans.
Diagram of Follicle Development
Primordial Follicle
│
▼
Primary Follicle
│
▼
Secondary Follicle
│
▼
Graafian Follicle
│
Ovulation
│
▼
Corpus Luteum
│
(No Pregnancy)
▼
Corpus Albicans
Role (Functions) of Follicles
Follicles play several important roles in female reproduction:
1. Protect the Egg
They surround the developing egg and protect it from damage.
2. Nourish the Egg
They provide nutrients and oxygen required for proper growth.
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3. Produce Hormones
Follicles produce estrogen, which:
Develops female secondary sexual characteristics.
Regulates the menstrual cycle.
Helps prepare the uterus for pregnancy.
After ovulation, the corpus luteum produces progesterone, which maintains the uterine
lining for implantation.
4. Help in Egg Maturation
Follicles ensure that only healthy eggs mature and become ready for ovulation.
5. Release the Egg
The mature Graafian follicle ruptures during ovulation, releasing the egg into the fallopian
tube.
Importance of Oogenesis
Produces female gametes (eggs).
Maintains chromosome number after fertilization.
Ensures genetic variation through meiosis.
Essential for human reproduction.
Supports normal menstrual and reproductive cycles.
Conclusion
Oogenesis is the lifelong process through which a female produces mature egg cells. It
begins before birth, pauses until puberty, and then continues monthly until menopause.
During this process, follicles act as the egg's protective and nourishing environment,
supplying nutrients, producing essential hormones like estrogen and progesterone, and
finally releasing the mature egg during ovulation. Thus, oogenesis and follicles work
together to ensure healthy egg development, successful ovulation, and the possibility of
fertilization and pregnancy.
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SECTION-B
3. Describe gastrulation.
Ans: Introduction
Gastrulation is one of the most important stages of embryonic development. It is the
process through which a simple embryo made of a single layer of cells changes into an
embryo with three different cell layers. These three layers later develop into all the organs
and tissues of the body. Because of this, gastrulation is often called the beginning of body
formation or the foundation of life.
A famous developmental biologist, Lewis Wolpert, once said:
"It is not birth, marriage, or death, but gastrulation which is truly the most important time
in your life."
This statement highlights how essential gastrulation is for the development of every human
being and most animals.
What is Gastrulation?
Gastrulation is the developmental process that occurs after the blastula stage and before
the neurula stage.
During gastrulation:
Cells of the blastula move and rearrange themselves.
A hollow ball of cells is transformed into a gastrula.
Three primary germ layers are formed:
1. Ectoderm (Outer layer)
2. Mesoderm (Middle layer)
3. Endoderm (Inner layer)
These layers are the building blocks of every organ in the body.
Why is Gastrulation Important?
Without gastrulation:
The body cannot develop properly.
Organs such as the brain, heart, lungs, muscles, liver, and kidneys would never form.
The embryo would not have a proper body plan.
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Therefore, gastrulation is considered the foundation of organ development.
Stages of Gastrulation
1. Formation of the Blastula
Before gastrulation begins, the fertilized egg divides repeatedly by cleavage.
The embryo develops into a hollow ball of cells called the blastula.
Outer cells surround a fluid-filled cavity known as the blastocoel.
2. Cell Movement Begins
At the beginning of gastrulation, cells start moving inward through a specific region.
This movement changes the simple blastula into a more complex embryo.
Different types of cell movements include:
Invagination cells fold inward.
Involution cells roll inside.
Ingression individual cells migrate inward.
Delamination one cell layer splits into two.
Epiboly outer cells spread over the embryo.
These movements help position cells in their correct locations.
3. Formation of the Three Germ Layers
After cell movement, three primary germ layers are established.
A. Ectoderm (Outer Layer)
The ectoderm forms structures that remain on the outside or develop into the nervous
system.
It gives rise to:
Brain
Spinal cord
Nerves
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Skin
Hair
Nails
Eye lens
Tooth enamel
Think of the ectoderm as the body's protective covering and control center.
B. Mesoderm (Middle Layer)
The mesoderm develops into the body's supporting and transport systems.
It forms:
Muscles
Bones
Cartilage
Heart
Blood
Blood vessels
Kidneys
Reproductive organs
Connective tissues
It acts as the framework and engine of the body.
C. Endoderm (Inner Layer)
The endoderm forms the internal lining of many organs.
It develops into:
Digestive tract
Liver
Pancreas
Lungs
Thyroid gland
Urinary bladder lining
It mainly forms the internal organs responsible for digestion and metabolism.
Simple Diagram of Gastrulation
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Blastula
___________________
/ \
| Blastocoel |
| |
\___________________/
↓
Cell movements begin
(Invagination, Ingression, etc.)
↓
Gastrula
-----------------
| Ectoderm | → Skin, Brain, Nervous System
|-----------------|
| Mesoderm | → Muscles, Bones, Heart, Blood
|-----------------|
| Endoderm | → Liver, Lungs, Intestine
-----------------
Functions of Gastrulation
Gastrulation performs several essential functions:
Converts the blastula into a gastrula.
Forms the three germ layers.
Establishes the basic body plan.
Determines the future position of organs.
Begins tissue and organ formation.
Creates the digestive cavity (primitive gut or archenteron in many animals).
Helps establish the body's head-to-tail and left-right axes.
Importance of the Germ Layers
Germ Layer
Major Structures Formed
Ectoderm
Brain, spinal cord, skin, hair, nails, eyes
Mesoderm
Muscles, bones, heart, blood, kidneys, reproductive organs
Endoderm
Digestive tract, liver, pancreas, lungs, thyroid, bladder lining
Easy Way to Remember
Think of building a house:
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Ectoderm = Roof and electrical system (protection and control).
Mesoderm = Walls, pillars, and pipes (support and transport).
Endoderm = Kitchen and plumbing inside (digestion and internal organs).
Just as every part of a house has a different purpose, each germ layer develops into a
unique set of organs that work together to form a complete body.
Conclusion
Gastrulation is a crucial stage of embryonic development during which a simple blastula
transforms into a gastrula through coordinated cell movements. This process produces the
three primary germ layersectoderm, mesoderm, and endodermwhich ultimately give
rise to every tissue and organ in the body. Gastrulation also establishes the basic body plan
and prepares the embryo for later stages such as organ formation. Because it lays the
foundation for the entire body, gastrulation is rightly regarded as one of the most
significant events in the development of all multicellular animals.
4. Explain embryonic development of Herdmania.
Ans: Embryonic Development of Herdmania (Simple Explanation)
Embryonic development means the process by which a fertilized egg (zygote) grows and
develops into a young larva. In Herdmania (a sea squirt or tunicate), this development is
indirect, because the embryo first becomes a free-swimming tadpole larva, which later
changes into the adult through metamorphosis.
Introduction
Herdmania is a marine chordate belonging to the subphylum Urochordata (Tunicata).
Although the adult looks like a simple sac attached to rocks, its embryo and larva show
important chordate characters such as a notochord, dorsal hollow nerve cord, and tail.
This is why the embryonic development of Herdmania is very important in zoology.
Think of its life like a butterfly:
Egg → Embryo → Tadpole Larva → Adult Herdmania
Just as a caterpillar changes into a butterfly, the larva of Herdmania changes into a
fixed adult by metamorphosis.
1. Fertilization
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Herdmania is hermaphrodite, meaning one animal has both male and female
reproductive organs.
However, self-fertilization is prevented.
Eggs and sperms are released into seawater.
External fertilization usually occurs in water.
When a sperm fuses with an egg, a zygote is formed.
Result: A single-celled zygote is produced.
2. Cleavage
The zygote now begins dividing repeatedly.
These rapid divisions are called cleavage.
Cleavage is holoblastic (complete) because the whole egg divides.
It is unequal and determinate, meaning each cell has a fixed future role.
The embryo passes through several stages:
2-cell stage
4-cell stage
8-cell stage
16-cell stage
Morula (solid ball of cells)
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Imagine cutting a cake into smaller and smaller pieces without increasing its size. The
embryo becomes a ball of many tiny cells.
3. Blastula Formation
As cleavage continues:
A hollow cavity called the blastocoel develops.
The embryo now becomes a blastula.
The blastula is simply a hollow ball of cells.
Think of it like a hollow rubber ball whose wall is made of living cells.
4. Gastrulation
Next comes one of the most important stages called gastrulation.
Some cells move inward through invagination.
This forms the gastrula, which has:
Ectoderm (outer layer) forms skin and nervous system.
Mesoderm (middle layer) forms muscles and connective tissues.
Endoderm (inner layer) forms digestive organs.
At this stage:
The primitive gut (archenteron) develops.
An opening called the blastopore is formed.
This stage lays the foundation for all future organs.
5. Organ Formation (Organogenesis)
Now different organs begin to develop.
Important structures formed include:
a) Notochord
Develops only in the tail region.
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Provides support during swimming.
b) Dorsal Hollow Nerve Cord
Forms above the notochord.
Develops into the nervous system.
c) Tail
Helps the larva swim freely in water.
d) Pharynx
Develops gill slits for future feeding.
At this stage, the embryo clearly shows all major chordate characteristics.
6. Formation of Tadpole Larva
Finally, the embryo develops into a tadpole larva.
Characteristics of Tadpole Larva
Oval body
Long muscular tail
Notochord in the tail
Dorsal hollow nerve cord
Adhesive papillae at the front
Sense organs for detecting light and gravity
The larva swims actively in seawater.
This larval stage resembles primitive chordates and proves that tunicates belong to the
phylum Chordata.
7. Metamorphosis
After swimming for some time:
The larva finds a suitable rock or hard surface.
It attaches itself using adhesive papillae.
A remarkable transformation called metamorphosis begins.
During metamorphosis:
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Tail disappears.
Notochord degenerates.
Dorsal nerve cord becomes very small.
Sense organs disappear.
The body rotates.
Large pharynx and siphons develop.
The animal becomes permanently attached (sessile).
Finally, it changes into the adult Herdmania.
This transformation is called retrogressive metamorphosis, because the active, complex
larva changes into a simpler adult.
Simple Flow Diagram
Egg
│
▼
Fertilization
│
▼
Zygote
│
▼
Cleavage
│
▼
Morula
│
▼
Blastula
│
▼
Gastrula
│
▼
Organogenesis
│
▼
Tadpole Larva
│
▼
Metamorphosis
│
▼
Adult Herdmania
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Important Features to Remember
Fertilization is external.
Cleavage is holoblastic, unequal, and determinate.
Embryo passes through morula, blastula, and gastrula stages.
Three germ layers (ectoderm, mesoderm, endoderm) are formed.
The larva possesses notochord, dorsal hollow nerve cord, and tail.
The adult loses most chordate characters during retrogressive metamorphosis.
The tadpole larva demonstrates that Herdmania is a true chordate.
Conclusion
The embryonic development of Herdmania begins with external fertilization, followed by
cleavage, blastula formation, gastrulation, organogenesis, and the formation of a free-
swimming tadpole larva. The larva possesses all the important chordate features, including
a notochord, dorsal hollow nerve cord, and muscular tail. Later, it undergoes retrogressive
metamorphosis, during which it attaches to a surface, loses its tail and notochord, and
develops into a sessile adult. Thus, although the adult appears simple, its embryonic
development clearly reveals its evolutionary relationship with other chordates.
SECTION-C
5. Give an account of development upto three germinal layers and their fate in chick.
Ans: Development up to Three Germinal Layers and Their Fate in Chick
The development of a chick embryo is one of the most interesting processes in biology. A
single fertilized egg gradually transforms into a complete chick with different organs,
tissues, and body systems. This process begins immediately after fertilization and continues
while the egg is incubated. One of the most important stages of embryonic development is
the formation of the three germinal (germ) layersectoderm, mesoderm, and endoderm.
These layers are called the primary germ layers because every organ in the chick's body
develops from them.
1. Fertilization and Early Development
Development starts when the sperm fertilizes the ovum (egg). The fertilized egg is called a
zygote.
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The zygote undergoes repeated cell divisions called cleavage. Since the chick egg contains a
large amount of yolk, cleavage is meroblastic (partial), meaning only the small disc of
cytoplasm at the animal pole divides. This forms a disc of cells known as the blastoderm.
The blastoderm then separates into two layers:
Epiblast (upper layer)
Hypoblast (lower layer)
A cavity forms between them called the blastocoel.
The epiblast gives rise to all three germ layers, while the hypoblast mainly helps in nutrition
and supports embryo formation.
2. Gastrulation Formation of Three Germinal Layers
The process by which the three germ layers are formed is called gastrulation.
During gastrulation:
A thickened line appears on the epiblast called the primitive streak.
Cells of the epiblast move inward through this primitive streak.
These migrating cells arrange themselves into three distinct layers.
These are:
(A) Ectoderm (Outer Layer)
The cells that remain on the surface form the ectoderm.
It is the outermost germ layer and mainly forms structures related to protection and the
nervous system.
Fate of Ectoderm
The ectoderm develops into:
Brain
Spinal cord
Nerves
Skin epidermis
Hair and feathers
Nails and claws
Beak covering
Lens of the eye
Inner ear
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Enamel of teeth
Easy Trick:
Ectoderm = External structures + Nervous system
(B) Mesoderm (Middle Layer)
The second group of migrating cells forms the mesoderm.
It lies between the ectoderm and endoderm and forms most supporting structures of the
body.
Fate of Mesoderm
The mesoderm develops into:
Muscles
Bones
Cartilage
Heart
Blood
Blood vessels
Kidneys
Gonads (testes and ovaries)
Connective tissues
Dermis of skin
Body cavity lining
Easy Trick:
Mesoderm = Muscles + Movement + Middle organs
(C) Endoderm (Inner Layer)
The first cells migrating inward replace the hypoblast and form the endoderm.
This is the innermost germ layer.
Fate of Endoderm
The endoderm develops into:
Lining of digestive tract
Liver
Pancreas
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Lining of respiratory tract (lungs)
Thyroid and parathyroid glands (partly)
Urinary bladder lining
Easy Trick:
Endoderm = Internal digestive and respiratory organs
Diagram Showing Formation of Germ Layers
ECTODERM
-----------------------
MESODERM
-----------------------
ENDODERM
-----------------------
YOLK
Formation During Gastrulation
Primitive Streak
↓
Cell Migration
ECTODERM
----------------------------
MESODERM
----------------------------
ENDODERM
----------------------------
YOLK
Fate of the Three Germinal Layers (Summary Table)
Germ Layer
Position
Main Structures Formed
Ectoderm
Outer
Brain, spinal cord, nerves, skin, feathers, eye lens, enamel
Mesoderm
Middle
Muscles, bones, heart, blood, kidneys, reproductive organs
Endoderm
Inner
Digestive tract, liver, pancreas, lungs, bladder lining
Importance of Germ Layers
The three germ layers are called the foundation of the embryo because every tissue and
organ originates from them. If these layers do not form properly, normal embryonic
development cannot occur. Their coordinated growth ensures that the chick develops a
functioning nervous system, circulatory system, digestive system, respiratory system,
skeleton, muscles, and skin.
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Conclusion
The development of a chick embryo begins with fertilization, followed by cleavage and the
formation of the blastoderm. During gastrulation, cells of the epiblast migrate through the
primitive streak to form the three germinal layersectoderm, mesoderm, and endoderm.
Each layer has a specific role: the ectoderm forms the nervous system and outer body
covering, the mesoderm forms muscles, bones, blood, heart, and reproductive organs, while
the endoderm forms the digestive and respiratory linings along with organs such as the liver
and pancreas. Together, these three germ layers act as the basic building blocks from which
the entire chick embryo develops into a healthy bird. This is why the formation and fate of
the three germinal layers is considered one of the most important events in embryology.
6. What are the steps of metamorphosis in frog ?
Ans: What are the Steps of Metamorphosis in Frog?
Metamorphosis is the amazing process by which a frog changes its body shape and
structure as it grows from an egg into an adult frog. The word "metamorphosis" means a
complete change in form or structure during the life cycle of an animal. Frogs do not look
like their parents when they are born. Instead, they go through several stages before
becoming fully developed adults. This transformation is controlled by a hormone called
thyroxine, which is produced by the thyroid gland.
A frog's life cycle is a perfect example of complete metamorphosis, where every stage has
its own special features and functions.
Simple Diagram of Frog Metamorphosis
Eggs
│
▼
Tadpole
│
▼
Tadpole with Hind Legs
│
▼
Tadpole with Front Legs
│
▼
Froglet
│
▼
Adult Frog
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Step 1: Egg Stage
The life of a frog begins as an egg. Female frogs lay hundreds or even thousands of eggs in
ponds, lakes, or other freshwater bodies. These eggs are covered with a jelly-like coating
that protects them from drying out and provides some protection from predators.
Inside each egg, a tiny embryo begins to grow. The embryo uses the nutrients stored in the
egg for its development. Depending on the temperature and environmental conditions, the
eggs usually hatch within one to three weeks.
Characteristics:
Found in water.
Protected by a jelly-like covering.
Embryo develops inside the egg.
No movement outside the egg.
Step 2: Tadpole Stage
When the egg hatches, a tadpole comes out. A tadpole looks more like a small fish than a
frog.
At this stage:
It has a long tail for swimming.
It breathes through gills, just like a fish.
It lives completely in water.
It mostly eats algae and other plant materials.
The tadpole spends most of its time swimming and feeding so it can grow rapidly.
Step 3: Tadpole with Hind Legs
After several weeks, the tadpole begins to change.
The first visible sign of metamorphosis is the appearance of hind legs.
During this stage:
Hind legs grow larger.
The body becomes stronger.
The lungs begin developing.
The tadpole still uses gills but slowly starts preparing for life on land.
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The tail is still long and helps in swimming.
Step 4: Tadpole with Front Legs
Next, the front legs appear.
Many important changes occur at this stage:
Both front and hind legs are fully developed.
Lungs become functional.
Gills gradually disappear.
The digestive system changes because the frog's diet changes from plants to insects.
The tail starts becoming shorter because the body absorbs its nutrients.
The tadpole now spends time both in water and near the water's surface.
Step 5: Froglet Stage
This stage is called the froglet.
The froglet looks almost like a small adult frog, but it still has a small tail.
Important changes include:
Tail becomes very short.
Lungs are fully developed.
It breathes air.
It starts catching small insects.
It begins hopping instead of only swimming.
The froglet can now survive both on land and in water.
Step 6: Adult Frog
Finally, the tail completely disappears, and the frog becomes an adult frog.
The adult frog has:
Four strong legs for jumping and swimming.
Well-developed lungs for breathing air.
Moist skin that also helps in respiration.
A long sticky tongue for catching insects.
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The ability to reproduce and lay eggs, beginning the life cycle again.
Adult frogs are carnivorous, feeding mainly on insects, worms, spiders, and other small
animals.
Major Changes During Metamorphosis
During metamorphosis, many body parts change dramatically:
Gills change into lungs, allowing the frog to breathe air.
Tail gradually disappears as its nutrients are absorbed by the body.
Four legs develop for walking, hopping, and swimming.
The digestive system changes from eating plants to digesting animals.
Eyes become larger and move to the top of the head, helping the frog see above
water.
The heart changes from a fish-like structure to a three-chambered heart suitable for
amphibians.
Importance of Metamorphosis
Metamorphosis helps frogs survive in different environments.
The tadpole lives in water and eats plants, reducing competition with adult frogs.
The adult frog lives both on land and in water and eats insects.
This allows frogs to use different food sources and habitats during different stages of
life.
It also helps maintain ecological balance by controlling insect populations.
Conclusion
Metamorphosis in frogs is one of nature's most fascinating transformations. A frog starts its
life as a tiny egg, hatches into a fish-like tadpole with gills and a tail, gradually develops legs
and lungs, loses its tail, and finally becomes a fully grown adult frog. Each stage is carefully
planned by nature and controlled by hormones, allowing the frog to adapt from an aquatic
(water) life to an amphibious (land and water) life. This remarkable life cycle demonstrates
how living organisms grow, adapt, and survive in changing environments.
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SECTION-D
7. How is placenta formed? What are its types and functions ?
Ans: 7. How is Placenta Formed? What are its Types and Functions?
Introduction
The placenta is a special organ that develops only during pregnancy. It acts as a lifeline
between the mother and the developing baby (foetus). The placenta supplies the baby
with oxygen and nutrients, removes waste products, and also produces hormones that help
maintain pregnancy.
You can think of the placenta as a "natural bridge" or "life support system" that connects
the mother and the baby without allowing their blood to mix directly.
How is the Placenta Formed?
Placenta formation begins soon after fertilization.
Step 1: Fertilization
A sperm fertilizes an egg in the fallopian tube.
The fertilized egg is called a zygote.
Step 2: Formation of Blastocyst
The zygote divides repeatedly while moving toward the uterus.
After about 56 days, it becomes a blastocyst, a hollow ball of cells.
Step 3: Implantation
The blastocyst attaches itself to the inner lining of the uterus (endometrium).
This process is called implantation.
Step 4: Development of Placenta
The outer cells of the blastocyst, called the trophoblast, grow into the uterine wall.
These cells form tiny finger-like projections called chorionic villi.
The chorionic villi become surrounded by the mother's blood vessels.
Together, the foetal chorion and the mother's uterine tissue (decidua basalis) form
the placenta.
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Important Point:
Although the mother's and baby's blood come very close together, they normally do not
mix directly. Nutrients, oxygen, and waste products pass through a thin membrane by
diffusion and active transport.
Simple Diagram of Placenta Formation
Mother's Uterus
_________________________
| |
| Placenta |
| ******************* |
| * Chorionic Villi * |
| ******************* |
| || |
| Umbilical Cord |
| || |
| Baby (Foetus) |
|_________________________|
Diagram Explanation:
Placenta: Attached to the wall of the uterus.
Umbilical cord: Connects the placenta to the baby.
Chorionic villi: Increase the surface area for exchange of nutrients and oxygen.
Types of Placenta
Placenta can be classified in different ways.
1. Based on Distribution of Chorionic Villi
A. Diffuse Placenta
Chorionic villi are spread over the entire surface.
Example: Horse and pig.
B. Cotyledonary Placenta
Villi are grouped into separate patches called cotyledons.
Example: Cow and sheep.
C. Zonary Placenta
Chorionic villi form a belt-like ring.
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Example: Dog and cat.
D. Discoidal Placenta
Chorionic villi are concentrated in one circular disc.
Example: Humans and monkeys.
Humans have a discoidal placenta.
2. Based on Histological Layers
Depending on how many tissue layers separate maternal and foetal blood:
Epitheliochorial
Syndesmochorial
Endotheliochorial
Hemochorial (found in humans)
In the hemochorial placenta, the chorionic villi are directly surrounded by maternal blood,
making the exchange of nutrients very efficient.
Functions of Placenta
The placenta performs many important functions.
1. Nutrition
Supplies glucose, amino acids, vitamins, minerals, fats, and water to the baby.
These nutrients help the baby grow properly.
2. Respiration
Oxygen from the mother's blood passes into the baby's blood.
Carbon dioxide from the baby passes back to the mother for removal.
Thus, the placenta acts like the baby's temporary lungs.
3. Excretion
The placenta removes waste products produced by the baby, such as:
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Carbon dioxide
Urea
Uric acid
Creatinine
These wastes enter the mother's blood and are removed by her kidneys and lungs.
4. Hormone Production (Endocrine Function)
The placenta produces several important hormones:
Human Chorionic Gonadotropin (hCG): Maintains early pregnancy and is detected in
pregnancy tests.
Progesterone: Maintains the uterine lining and prevents miscarriage.
Estrogen: Supports growth of the uterus and prepares the breasts for breastfeeding.
Human Placental Lactogen (hPL): Helps prepare the mother's body for milk
production and supports foetal growth.
5. Protection
Acts as a partial barrier against many harmful bacteria and substances.
Transfers maternal antibodies (IgG) to the baby, giving temporary immunity after
birth.
However, some viruses, alcohol, nicotine, and certain medicines can still cross the placenta.
6. Storage Function
The placenta stores nutrients such as:
Glycogen
Iron
Fat
Vitamins
These reserves support the baby's development.
7. Exchange of Gases and Nutrients
The placenta allows the exchange of:
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Oxygen
Carbon dioxide
Nutrients
Water
Electrolytes
Hormones
without direct mixing of maternal and foetal blood.
Why is the Placenta Important?
Without the placenta:
The baby would not receive oxygen.
Nutrients could not reach the foetus.
Waste products would accumulate.
Pregnancy could not continue normally.
This is why the placenta is often called the "organ of pregnancy" or the "life-support
system of the foetus."
Key Points for Exam
Placenta is a temporary organ formed during pregnancy.
It develops from the chorion of the embryo and the uterine lining (decidua basalis)
of the mother.
Humans have a discoidal, hemochorial placenta.
It connects the foetus to the mother through the umbilical cord.
Maternal and foetal blood do not normally mix directly.
Main functions include:
o Nutrition
o Respiration
o Excretion
o Hormone secretion
o Protection
o Storage of nutrients
o Transfer of antibodies
Conclusion
The placenta is one of the most remarkable organs in human biology because it forms only
during pregnancy and performs many life-saving functions for the developing baby. It
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originates from both the foetal chorion and the mother's uterine tissue, creating a strong
connection through the umbilical cord. Acting as the baby's lungs, kidneys, digestive
system, hormone-producing gland, and protective barrier, the placenta ensures that the
foetus receives everything needed for healthy growth until birth. Because of these vital
roles, it is rightly known as the lifeline of the developing baby.
8. Explain embryonic development of rabbit.
Ans: Embryonic development is the process by which a single fertilized egg (zygote) grows
and develops into a fully formed baby rabbit. Rabbits are viviparous mammals, which
means the baby develops inside the mother's uterus (womb) and receives nourishment
directly from the mother through the placenta. This development takes about 3031 days.
1. Fertilization
The process begins when a sperm (male reproductive cell) meets an ovum or egg (female
reproductive cell) inside the fallopian tube (oviduct).
The sperm and egg fuse together.
This fusion forms a zygote, which is the first cell of the new rabbit.
The zygote contains genetic material from both parents and determines all the
characteristics of the baby rabbit.
Remember:
Sperm + Egg = Zygote
2. Cleavage
After fertilization, the zygote starts dividing repeatedly through mitosis. These rapid
divisions are called cleavage.
During cleavage:
The size of the embryo does not increase.
Only the number of cells increases.
First, the zygote divides into 2 cells, then 4 cells, 8 cells, 16 cells, and so on.
This stage helps produce many small cells called blastomeres.
3. Morula Stage
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After several rounds of cleavage, the embryo becomes a solid ball of about 1632 cells
known as the morula.
The morula resembles a small mulberry, which is why it gets this name ("morula" means
mulberry).
At this stage:
Cells are tightly packed.
The embryo continues moving toward the uterus.
4. Blastocyst Stage
As more cell divisions occur, fluid enters the morula, forming a hollow structure called the
blastocyst.
The blastocyst has three important parts:
Outer layer (Trophoblast): Forms the placenta.
Inner Cell Mass (Embryoblast): Develops into the baby rabbit.
Blastocoel: Fluid-filled cavity.
The blastocyst reaches the uterus and prepares to attach to its wall.
5. Implantation
The blastocyst attaches itself to the inner lining of the uterus. This process is called
implantation.
After implantation:
The embryo becomes firmly attached to the mother's uterus.
Nutrients and oxygen begin reaching the embryo.
Waste products from the embryo are removed through the mother's blood.
This attachment is very important because it allows the embryo to survive and grow.
6. Formation of Germ Layers (Gastrulation)
The embryo now develops three primary germ layers. These layers later form every organ
of the body.
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Germ Layer
Forms
Ectoderm
Skin, hair, nails, brain, spinal cord, nervous system
Mesoderm
Muscles, bones, heart, kidneys, blood, reproductive organs
Endoderm
Lungs, liver, pancreas, stomach and intestinal lining
This stage is called gastrulation.
7. Organ Formation (Organogenesis)
Now the organs begin to develop.
Some important developments include:
Brain and spinal cord
Heart starts beating
Eyes and ears appear
Limbs begin to grow
Digestive system develops
Lungs and liver are formed
This stage is known as organogenesis.
8. Formation of Placenta
The placenta is a temporary organ formed between the mother and the developing embryo.
Its functions are:
Supplies oxygen.
Provides nutrients.
Removes carbon dioxide.
Removes waste materials.
Transfers some antibodies for protection.
The blood of the mother and embryo does not mix directly, but substances pass through
the placental membrane.
9. Development of Extra-Embryonic Membranes
The embryo is protected by four membranes:
Amnion: Surrounds the embryo with amniotic fluid and protects it from shocks.
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Chorion: Helps form the placenta.
Allantois: Stores waste in early development and helps in gas exchange.
Yolk Sac: Provides nutrients during the early stages.
These membranes protect and support the embryo throughout pregnancy.
10. Foetal Development and Birth
After all organs are formed, the embryo is called a foetus.
During this period:
Body size increases.
Bones become stronger.
Fur begins to develop.
Eyes, ears, and limbs mature.
The lungs become ready for breathing.
After about 3031 days of gestation, the baby rabbit is fully developed and is born through
the birth canal.
Simple Flow Diagram
Sperm + Egg
│
▼
Zygote
│
▼
Cleavage
(2 → 4 → 8 → 16 Cells)
│
▼
Morula
│
▼
Blastocyst
│
▼
Implantation
│
▼
Gastrulation
(Three Germ Layers)
│
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▼
Organogenesis
│
▼
Placenta Formation
│
▼
Foetus
│
▼
Birth of Baby Rabbit
Key Points for Exams
Rabbit is a viviparous mammal.
Fertilization occurs in the fallopian tube (oviduct).
The fertilized egg is called a zygote.
Cleavage produces the morula and then the blastocyst.
The blastocyst undergoes implantation in the uterus.
Three germ layers (ectoderm, mesoderm, and endoderm) give rise to all body
organs.
Organogenesis forms the major organs.
The placenta supplies nutrients and oxygen and removes wastes.
The embryo becomes a foetus, grows inside the uterus for about 3031 days, and is
then born as a young rabbit.
In simple words: Embryonic development in a rabbit is a continuous journey that starts with
a single fertilized cell (zygote). Through repeated cell divisions, implantation, formation of
germ layers, development of organs, and nourishment from the placenta, this tiny cell
gradually transforms into a complete baby rabbit ready to be born. This process
demonstrates how complex life can develop in an organized and fascinating manner from
just one cell.
This paper has been carefully prepared for educational purposes. If you notice any mistakes or
have suggestions, feel free to share your feedback.
