paper 1

Question 1

Gastrulation

Answer 1

The gastrulation is the most characteristic event occurring
during the third week of gestation because it forms
the three germ layers that form all the tissues and organs
of the body. The process of formation of three germ layers
is called gastrulation.
The three germ layers are formed as follows.
1. First, the embryoblast differentiates into two layers:
(a) a superficial layer consisting of flat cells called
endoderm and (b) deep layer consisting of columnar
cells called ectoderm. The embryo at this
stage is termed bilaminar embryonic disc.
2. Now the ectoderm forms a linear thickening in the
midline of embryonic disc called primitive streak.
The primitive streak gives rise to third layer—
the mesoderm—that lies between ectoderm and
endoderm. The embryo at this stage is called trilaminar
embryonic disc. During the formation
of the three germ layers other embryonic structures
also develop. The details of formation of three germ
layers along with other embryonic structures are
described in the following text.

Question 2

notocord formation

Answer 2

The cells of primitive knot of the primitive streak
proliferate to form prenotochordal cells that move
inward (i.e., invaginates) to produce a central
depression called blastopore.
From the bottom of the blastopore, the prenotochordal
cells of the primitive knot migrate forward
in the midline between the ectoderm and the endoderm
of the bilaminar germ disc to form a solid
cord of cells called notochordal process or head
process.
2. The notochord increases in length by extending
caudally as the primitive streak recedes caudally
and regresses.
The notochordal process gets canalized to form
notochordal canal. The notochordal canal is continuous
with the blastopore.
3. The floor of notochordal canal fuses with the
endoderm.
4. Later both the fused parts (wall of notochordal
canal and part of endoderm) breakdown. The notochordal
canal now communicates with the yolk sac
at one end and with the amniotic cavity at the
other end. At this stage, the amniotic cavity and
the yolk sac are in communication with each other.
5. Gradually the walls of the canal become flattened
to form a flattened plate called notochordal plate.
6. The notochordal plate becomes curved to form a tube.
7. The proliferation of cells of the tube converts
it into a solid cord of cells to form definitive notochord.
The endoderm is restored, and now the
notochord is completely separated from the
endoderm.
The notochord is present in all the animals belonging
to phylum Chordata.
2. In human beings, it appears only in embryo. In later
life, it disappears but its remnants are seen in the form
of nucleus pulposus of the intervertebral discs and
apical ligament of dens of second cervical vertebra.

Question 3

neural tube

Answer 3

The process of formation of neural tube is called neurulation.
The neural tube is formed from the ectoderm
overlying the notochord as follows:
1. The cells of ectoderm overlying the notochord get
differentiated into specialized cells called neuroectodermal
cells.
2. The neuroectodermal cells proliferate to form a
thick plate called neural plate. The neural plate
extends in midline from prochordal plate to the
primitive knot.
3. The margins of the neural plate get elevated (neural
folds) as paraxial mesoderm proliferates on either
side of the notochord. This leads to the formation of neural groove flanked by neural folds. The neural
groove becomes deeper and neural folds move
towards midline to fuse with each other to form a
cylindrical neural tube. The fusion of neural folds
(i.e., closure of neural tube) begins in the middle
and gradually extends in cephalic and caudal directions.
During the closure of the neural groove, the
cells at the tips of neural folds (neural crests) do
not take part in the formation of neural tube.
When the surface ectoderm is restored they form
bilateral masses dorsolateral to the neural tube,
deep to surface ectoderm. The cranial part of neural
tube enlarges and forms the brain while the caudal
part of neural tube remains tubular and forms the
spinal cord.

Question 4

intraembryonic mesoderm

Answer 4

Paraxial mesoderm: The mesoderm lateral to neural
tube condenses to form the paraxial mesoderm.
2. Intermediate mesoderm: The mesoderm in
between the paraxial mesoderm and lateral plate
mesoderm is termed intermediate mesoderm. It is
not only intermediate in position but also intermediate
in thickness as compared to paraxial and lateral
plate mesoderms.
3. Lateral plate mesoderm: The mesoderm in the
lateral part of embryonic disc remains thin and
forms lateral plate mesoderm.

Question 5

kidney

Question 6

pancreas

Question 7

primitive streak

Question 8

paramesonephric duct

Question 9

mesonephric duct

Question 10

uterus

Question 11

testis

Question 12

mid gut roation

Question 13

somites

Question 14

meckles diverticulum

Question 15

vitellointestinal duct