46 TEXT-BOOK OF EMBRYOLOGY. 



meroblastic cleavage as seen in the eggs of Birds, it becomes immediately evi- 

 dent that the differences between them are explainable entirely by reference to 

 the greater quantity of yolk in the bird's egg. The real activity of segmenta- 

 tion is in both cases confined almost wholly to the protoplasm. In the frog's 

 egg the amount of yolk present is sufficient to impede segmentation in the 

 larger cells but not to prevent it. In the bird's egg the amount of yolk is so 

 great that it cannot be made to undergo complete segmentation. 



Reviewing the results of cleavage, it is to be noted that in every case there is 

 formed a larger or a smaller group of cells. In the case of equal holoblastic 

 cleavage, these cells are all of the same or of nearly the same size, and constitute 



Micromeres. 



mz 



Macromeres. 



FIG. 30. From a sagittal section through blastula of frog. Bonnet, 

 mz., Marginal zone. 



what is known as the morula or mulberry mass (Fig. 25, E). A similar condition 

 obtains in unequal holoblastic cleavage with the one exception, that there is a 

 marked difference in the size of the cells constituting the morula (Fig. 27). In 

 superficial meroblastic cleavage the group of cells forms a layer enclosing the 

 central yolk, the latter being unsegmented but containing some nuclei. In 

 discoidal meroblastic cleavage the group of cells spreads itself over a limited 

 superficial area, while beneath it lies the large mass of unsegmented yolk, con- 

 taining, however, some nuclei (Figs. 28 and 29). 



In holoblastic cleavage the blastomeres in the interior of the mass become 

 more or less separated during segmentation, a cavity thus being formed within 

 the so-called morula. This cavity increases in size, the cells being pushed 

 centrifugally, and the embryo soon consists of a layer or layers of cells enclosing 



