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This thread is related to my previous thread which is still unsolved mainly. I need to be able to compare chick and human cleavage and blastulation of zygote. My friend says that

Chicken have discoblast, human not. That is the main thing to get the idea of the difference.

I know that chicken have yolk sac with yolk, while human have yolk sac without yolk.

My notes say that

Cleavage in birds is partial discoidal (embryo forms disc and cells called blastodisc are on top of the yolk).

So embryo forms some disc, lets call it #1. Some cells called blastodisc are on top of the yolk. So are the cells called blastodisc on top of the yolk and the disc #1?

What is the name of the given disc that chick embryo forms? It seems that human is not forming the given disc. Apparently, the name of the given disc is discoblast, the thing he says is very important.

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It is a mnemonic of two words: discoidal + blastula = discoblastula. So it suggests that cleavage is partial discoidal in birds. Embryo forms disc. Cells called blastodisc are on top of the yolk. It seems that the formed disc is blastodisc.

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The cleavage in birds is restricted to the blastodisc lying on the top of the yolk towards the animal pole of the zygote, this type of cleavage is termed as discodial cleavage because the embryo forms disc and cells that are also on the top of the yolk.

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