Question

why is cleavage in birds meroblastic and discoidal

Answer 1

meroblastic : lines of segmentation do not completely pass through egg & remain confined to a part of egg. megalecicithal : very large amount of yolk. discoidal : disc of cells is produced at animal pole of zygote. as birds has megalecicithal eggs, the yolk provides resistance to cleavage, hence the cleavage is meroblastic & discoidal. meroblastic cleavage occurs in insects & reptiles also but it is not discoidal , it is suferficial ( having centrolecithal eggs).

Answer 2

Birds undergo discoidal meroblastic cleavage.

EXPLANATION:

The cleavage occurs in a small disc called blasto disk of cytoplasm 2-3m in diameter at the animal pole of egg shell. Initially, the 'cleavage' furrow appears "centrally" in the blasto disk and sequentially other cleavage layer follows to create single layered blastoderm.

The blastoderm is divided in to "equatorial and vertical cleavages" resulting the tissue with five to six layers of cell. In case of chick egg, the fertilization occurs in oviduct. Thereafter, "equatorial and vertical cleavages" divide the 'blastoderm' into a tissue 'five' to 'six' thick cell layers.

Fertilization of the chick egg occurs in the oviduct. The egg is consisting of small 'disc of cytoplasm' sitting atop a 'large yolk' forming "telolecithal".