Cleavage
Cleavage is the continuous process of segmentation of zygote
by the mitotic cell division. Cleavage of frog is unequal holoblastic in nature.
It takes place in both yolky and non yolky part. The size of new formed cells
are not equal hence cleavage of frog is unequal holoblastic.
The first cleavage in frog is vertical and it starts from
animal pole and passes towards vegetal pole.it divides zygote equally into two
parts forming 2-celled stage. The second cleavage is perpendicular to the first
cleavage.and divide zygote to 4-celled stage. The third cleavage occurs above
the equator to divide zygote into 8-celled stage. The cells lying above third
cleavage are called micromeres and below are called macromeres .
Blastula:
During continuous cleavage, a fluid filled cavity appears in
the animal pole region. This cavity is called blastocoel . Formation of blastocoel indicates the
formation of blastula stage. Morulation in frog is absent according to some
scientist, hence blastulation stage starts from 8-celled stage. Fluid in blasto
coel is albuminous in nature. This fluid is secreted by surrounding micromeres.
Blastocoel is a blind cavity i.e. without any openings. Roof of blastocoels
contains micromeres and base contains macromeres.blastula stage is stained by
intra vitum technique so that its cells are not killed. After intra vitum
staining we can see presumptive stages. Almost all animal pole represent
presumptive ectoderm. It is differentiated into presumptive epidermis and
neural plate. Cells of grey crescent region represent presumptive notochord and
presumptive mesoderm. Almost all vegetal pole represent presumptive endoderm.
Gastrulation:
It is the movement and
rearrangement of cells of blastula stage. It completes in following steps:
i)
Epiboly:
epiboly is the process of overgrowth of micromere. In this sage, the division
of micromere is faster and megamere is slower because micromere is nonyolky and
megameres are yolky. Due to the continuous rapid division of micromere, the
number of micromere becomes extremely more than megamere. Hence overflow of micromeres
towards megameres take place and finally micromere covers megamere externally leaving
a small uncovered megamere region called yolk plug.
ii)
Imboly: it is early stage of gastrular cell
migration. The special feature of Imboly is formation of blastopore. Blastopore
is a small cavity in region of presumptive notochord. Blastopore consists of dorsal
and ventral lips. In future, blastopore changes into large cavity called
archenteron.
iii)
Migration
of micromeres: it is also called as actual yolk plug stage. In this stage
micromeres starts flowing towards megameres near blastopore region. Due to this
a cavity forms inside embryo known as archenteron. As the size of archenteron increases,
size of blastocoels decreases. Due to inward migration of micromeres, pressure
is developed in megameres region, thus some of megameres cell come out through
blastopore forming a small mass known as yolk plug. Finally, micromeres
surround the whole embryo to form a layer called ectoderm. The micromeres at
corner of archenteron are mesoderm and start to form notochord. The megameres
situated in floor of archenteron modify into endoderm. Thus embryo consist iof
distinct two layers at this stage i.e. ectoderm and endoderm. Mesoderm is not
clearly differentiated from endoderm. Hence, embryo at this stage is
diploblastic.
iv)
Rotation
of gastrula: in blastula stage, blastocoel was larger in size which is subsequently
reduced in gastrula while archenteron is formed. As a result, location of
cavity changes in embryo ,thus centre of gravity of embryo also changes and
finally embryo rotates through 90 degree and plastopore reaches to posterior
region of embryo. Blastopore in future forms anus and blastocoels disappears.
It indicates the end of gastrulation.
Formation of
three germinal layers:
During gastrulation, embryo was diploblastic i.e.
outer ectoderm and inner endoderm was only found. After complete gastrulation,
mesoderm develops between ectoderm and endoderm. The embryo finally becomes triploblastic.
These three layers are called germinal layers as various tissues and organ
develop from these layers in future. The ectoderm is made up of micromeres and
endoderm is made up of megameres. Mesoderm is made up of both micro and micromeres.
Fate of three
germinal layers:
Ectoderm:
it gives rise to:
Epidermis of skin, enamel of teeth
Nervous system, pituitary gland and pineal body
Medulla of adrenal gland
Cornea, conjunctiva, lens and retina of eye
Sensory part of ear
Lining of buccal cavity and chloaca.
Endoderm:
it gives rise to:
Muscles, blood, dermis of skin
Bone, cartilage
Notochord
Heart, blood vessels, sclera and choroid of eye
Kidney, testes and ovary
Mesoderm:
it gives rise to:
Lining of alimentary canal from esophagus to rectum
Liver, pancrease
Trachea, lungs, bronchi
Urinary bladder, Adrenal cortex
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