4io 



COMPARATIVE ZOOLOGY 



into very small portions (cells) surrounding a central 

 cavity. This stage is known as the " mulberry mass," 

 or blastula (Fig. 361, c). If the yolk is larger, relatively 

 to the germinal vesicle, the process of division may go 

 on more slowly in one of the two parts of the egg, first 

 formed ; or in very large eggs, like those of birds and 

 cuttlefishes, only a small part of the yolk subdivides. 



In some form, the process of segmentation is found in 

 the eggs of all animals, as is also the following stage. 

 This step is the differentiation of the single layer of 

 cells into two parts, one for the body wall, the other for 

 the wall of the digestive tract. In 

 the typical examples, this ' is ac- 

 complished by one part of the 

 wall of the blastula turning in so 

 far as to convert the blastula into 

 a sort of double-walled cup, the 

 gastrula (Fig. 362). One half of 



FIG. 362. - Diagram of Gastrula \ f * ' 



of a worm ($*#):, prim- the wall of the blastula is now the 

 outer wall of the germ, the other 



body cavity ; en, endoderm ; foalf t h at Q f ^ Q digestive Cavity I 

 ec, ectoderm. 



the original blastula cavity is now 



the body cavity, the new cavity formed by the infold- 

 ing is the stomach and its opening is both mouth and 

 vent (Fig. 362). Some adult animals are little more 

 than such a sac. Hydra (Fig. 18), for instance, is 

 little different from a gastrula with tentacles, and one 

 of its relatives wants even these additions. 



Ordinarily, however, development goes much further. 

 From the two original layers arises, in various ways, a 

 third between them, making the three primitive germ 

 layers epiblast, mesoblast, and hypoblast. This new 

 layer is necessarily in the primitive body cavity, which 

 it may fill up ; or usually a new body cavity is formed, 

 in different ways in different groups. In by far. the 



