54 TEXT-BOOK OF EMBRYOLOGY. 



these are rounded off and the space here is somewhat augmented. In the 

 interior of the mass there is therefore a small cavity which, since the upper 

 four cells are smaller than the lower, is eccentric. As the blastomeres con- 

 tinue to divide around it, the cavity increases in size but remains eccentric. 

 During the first few divisions there is only a single layer of cells around the 

 cavity; then some of the cells divide parallel to the surface and a double 

 layer appears and then several layers. The multiplicity of layers is espe- 

 cially characteristic of the yolk cells. The entire structure is a hollow sphere 

 called the blastula and the eccentric cavity within, known as the blasto- 

 ccel or segmentation cavity, has a dome-shaped roof of micromeres and a 

 floor of macromeres (Fig. 29). The peripheral stratum of closely compacted 



Micromeres 



Macromeres 

 FIG. 29. From a sagittal section through blastula of frog. Bonnet, mz., Marginal zone. 



cells is the most highly pigmented while the cells beneath are less pig- 

 mented and somewhat more loosely arranged. The blastula is about the 

 same size as the egg before it began to divide. It is similar to Amphioxus 

 in that it is a hollow sphere, but is different in that the blastoccel is eccen- 

 tric and the cells form several layers instead of one. (Compare Figs. 20 

 and 29.) As the cells multiply, those in the highest part of the dome-like 

 roof of the blastoccel migrate toward the equator so that the roof becomes 

 thinner and the lateral wall becomes thicker. The thicker lateral wall, 

 which also exhibits rapid cell proliferation, is called the germ ring and prob- 

 ably corresponds to a similar zone of rapidly dividing cells in Amphioxus at 

 the beginning of gastrulation. On the side of the blastula where the gray 

 crescent is situated the germ ring migrates across the equator and down about 



