198 



COMPARATIVE ZOOLOGY. 



until the whole yolk is subdivided into very small por- 

 tions (cells) surrounding a central cavity. This stage is 

 known as the " mulberry-mass," or Uastula (Fig. 165, c). 



FIG. 165. First Stages in Segmentation f a Mammalian Egg: A, first division into 

 halves, with spermatozoa around it; and C, progressive subdivision, ultimate- 

 ly transforming the vitellus, or yolk, into a " mulberry mass" of globules, or em- 

 bryo-cells. 



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 Cuttle-fishes, 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 example*, this is accom- 

 plished by one part of the wall of 

 . i66._ Diagram of Gastra- the blastula turning in, so far as to 



la of a Worm (Sagitta): a, .1 , T , . , / 



primitive mouth ; 6, primi- convert the blastula into a sort of 

 XSR&E& -! double-walled cnp, the gastrula (Fig. 



endoderm : ec, ectoderm. 166). One half of the Wall of the 



blastula is now the outer wall of the germ, the other half 

 that of the digestive cavity ; the original blastula-cavity 

 is now the body-cavity, and the new cavity formed by the 

 infolding is the stomach, and its opening is both mouth 



