50 TEXT-BOOK OF EMBRYOLOGY. 



due to separation of its cells. This cavity is at first small but increases in size, 

 and, the cells being pushed out centrifugally, the embryo soon consists of a 

 layer or layers of cells enclosing a cavity. The cavity is known as the seg- 

 mentation cavity, while the entire embryo is known at this stage as the blastula. 



The simplest type of blastula is seen in Amphioxus, where it consists of a 

 nearly spherical segmentation cavity surrounded by a single layer of cells. 

 Some of the cells those which are more ventral and contain the larger 

 amount of yolk are slightly larger than others (Fig. 26, 6). 



In eggs in which the cells resulting from segmentation show greater in- 

 equality in size (due to difference in yolk content), as in the frog, the segmenta- 

 tion cavity is surrounded by several layers of cells. In such a blastula the roof 

 of the cavity is comparatively thin, being composed of small cells containing 



Micromeres. " 



Macromeres. 



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

 mz., Marginal zone. 



little yolk, micromeres, while the floor of the cavity is thick, being composed 

 of large yolk cells, macromeres. So thick is this wall of the vegetative pole of the 

 blastula that the large yolk cells extend into the segmentation cavity compress- 

 ing it into a crescentic cleft (Fig. 30). In the frog the roof of the segmentation 

 cavity is sharply defined from the floor, due to the fact that the outer layer of 

 cuboidal roof cells is densely pigmented. The rather sharply defined zone of 

 transition between pigmented micromeres and nonpigmented macromeres is 

 known as the marginal zone. 



In discoidal segmentation, the segmentation cavity is a mere slit between the 

 superficial protoplasmic cells and the underlying unsegmenting yolk with its 

 yolk nuclei (Fig. 29). Comparing it with unequal holoblastic cleavage, these 

 partially divided yolk cells which form the floor of the segmentation c'left in 



