40 



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 



r |G. 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 

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

 The third cleavage plane is also vertical, dividing two of the 

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

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

 ieir bases lying peripherally and toward the yolk mass. They are 

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



