CLEAVAGE. 45 



determine segmentation of the protoplasm, the number of protoplasmic seg- 

 ments corresponding to the number of nuclei. There is thus formed a super- 

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



(B) DISCOIDAL. This type of cleavage occurs in eggs which have an ex- 

 cessive amount of yolk and in which the protoplasm is confined to a small super- 

 ficial germ disk. The telolecithal ova of Birds furnish typical examples of this 

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

 plasmic 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 peripherally with the 



y.s. g.a. s.c. w.y. 



FIG. 29. From a vertical section through the germ disk of a fresh-laid hen's egg. Duval, Herturig. 

 g.d., Upper layer of germ disk; s.c., segmentation cavity; w.y., white yolk (see Fig. 7); y.s., lower 

 layer of germ disk (yolk cells, merocytes). 



underlying yolk mass and consequently with each other. The analogy be- 

 tween this condition and that described for the frog's egg is complete with the 

 one exception that in the latter the cleavage furrows cut completely through 

 the yolk cells or the yolk-containing portions of the cells, while in the bird's egg 

 the amount of yolk is so great that the cleavage furrow merely passes a short 

 distance into it without completely dividing it into segments. The fourth 

 cleavage plane is tangential, cutting off the apices of the six pyramidal segments. 

 The germ disk after the fourth cleavage thus consists of six small superficial 

 central cells and six larger cells which surround the small cells and also separate 

 the latter from the underlying yolk. From this point radial and tangential 

 cleavages follow each other without any semblance of regularity. The result 

 is a mass of small cells lying at the center of the disk and surrounded by larger 

 cells (Fig. 28, c, d). The smaller cells are completely separated from the under- 

 lying yolk while the larger cells are for a time continuous with it (Fig. 29). 

 Comparing the unequal holoblastic cleavage of the frog's egg with discoidal 



