cleavage in chick is called discoidal type why?
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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 the chick is called discoidal because it is limited to the blastodisc.
EXPLANATION:
Post-fertilization changes in the chicken egg are caused by several mitotic divisions. The entire egg at a point of development is divided into two poles, the vegetal pole and the animal pole.
The vegetal pole is responsible for storage of nutrition for the developing embryo, whereas the animal pole is responsible for further division to lead to the formation of the embryo.
The axis between the two poles is called a cleavage. In the chicken embryo, the cleavage itself is restricted to the blastodisc, a discoidal blastula that is formed after repeated mitotic division of the zygote. Therefore, the cleavage in chick is called as discoidal
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