Division of zygote in frog


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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