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TEXT-BOOK OF EMBRYOLOGY. 



parently determine segmentation of the protoplasm, the number of protoplasmic 

 segments corresponding to the number of nuclei. There is thus formed a 

 superficial layer of cells (blastomeres) enclosing the central nutritive yolk. 



(b) Discoidal. — This type of cleavage occurs in eggs which have an 

 excessive amount of yolk and in which the protoplasm is confined to a small 

 superficial germ disk. The telolecithal ova of Birds furnish typical examples 

 of this form of cleavage. The first cleavage plane is vertical and divides the 



c d 



Fig. 28. — Cleavage in hen's egg. Coste. Germinal disk and part of yolk, seen from above. 



protoplasmic disk into halves. The second cleavage plane is also vertical 

 and at right angles to the first, resulting in four approximately equal cells 

 (Fig. 28, a). The third cleavage plane is also vertical, dividing two of the 

 four cells (Fig. 28, b). The germ disk at the end of the third cleavage consists 

 of six pyramidal cells lying with their apices together in the center of the germ 

 disk, their bases lying peripherally and toward the yolk mass. They are 

 separated from one another at the surface, but are still continuous below and 



