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 blastula (Fig. 165, c). 
ABC 
Fig. 165.— First Stages in Segmentation of a Mammalian Egg: A, first division into 
halves, with spermatozoa around it; B aud C , progressive subdivision, ultimate¬ 
ly transforming the vitellus, or yolk, iuto 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 examples, this is accom- 
_____ plished by one part of the wall of 
Fig. 166.—Diagram of Gastru- the blastula turning in, so far as to 
la of a Worm (Sagitta): a, , ,, , , , ,. 
primitive mouth; b, primi- convert tlie blastula into a sort of 
pHmifivf b fc ody-ca a vUy y ; ; en t double-walled cup, the gastrula (Fig. 
endoderm; ec, ectoderm. 166). One half of tile 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 
