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Fertilization of frog eggs|Frog embryology|Immune system

Posted by Mumtaz khan Sunday, 20 May 2012

 Frog Embryology

Fertilization in case of frog takes place by the process of pseudocopulation because there is no external copulatory organ present in case of male and the female are in close proximity during the process it has the name pseudocopulation.

The egg gets surrounded by many sperms and there is specific reaction called as fertilizin-antifertilizin reaction between the two.(Fertilizin is present in the jelly layer of the egg and anti-fertilizin is secreted by the acrosome of the sperm).The sperm traces its path through the jelly layer and gets attached to the vitelline membrane.
The next reaction which takes place is acrosomal reaction followed by cortical action after which the vitelline envelope is converted into thick fertilization membrane.This prevents polyspermy.As soon as the sperm enters the ova(i.e.,the nucleus of sperm enters the egg) the second meiotic division is completed resulting in the release of the second polar body.
The space between the fertilization membrane and PM of the egg is called as peristelline space which is filled with the perivitelline fluid. In this fluid,the egg can rotate freely and also this space allows free exchange of gases.

The egg pronucleus and the female pronucleus fuse together to form the zygotic nucleus,this process is known as Amphimixis or Syngamy.
The entry of the sperm causes formation of a Grey crescent (which is a characteristic of an amphibian embryo);caused due to mixing of cytoplasmic granules or in other words due to the cytoplasmic movement because of the sperm entry.Grey crescent materials the functions as an organizer because if it is removed from the embryo,the embryo fail to develop further. At the same time if a normal embryo is grafted with another Gery crescent then two embryos develop.

Thus,formation of grey crescent fixes up final symmetry of the egg and future embryo.Also grey crescent marks the future dorsal side of the embryo.The sperm penetrates the egg perpendicular to the cortex and after penetration moves perpendicularly along the radius of the egg.This path of sperm in the egg cortex, is called as penetration path.After crossing the cortex,the sperm changes its direction and moves towards the egg nucleus.This changed path is also copulation path because it leads to the copulation (fusion) of the sperm pronuclei.
After fusion of both male and female pronuclei, a zygote nucleus is formed i.e.,a zygote will be formed
Further development of zygote proceeds with its cleavage and formation of blastula,gastrula and so on


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