82 ANIMAL LIFE 
her (varying with the various species or groups of animals) 
of cells is produced (Fig. 39, @). The phenomenon of re- 
peated division of the germ cell, and usually the surround- 
ing yolk, is called cleavage, and this cleavage is the first 
stage of development in the case of all many-celled animals. 
The first division of the germ cell produces usually two equal 
cells, but in some of the later divisions the new cells formed 
may not be equal. In some animalsall the cleavage cells are 
of equal size; in some there are two sizes of cells. The germ 
or embryo animal consists now of a mass of few or many 
undifferentiated primitive cells lying together and usually 
forming a sphere (Fig. 39, e), or perhaps separated and scat- 
tered through the food yolk of theegg. The next stage of de- 
velopment is this: the cleavage cells arrange themselves so 
as to form a hollow sphere or ball, the cells lying side by side 
to form the outer circumferential wall of this hollow sphere 
(Fig. 39, f). This is called the dlastula or blastoderm stage 
of development, and the embryo itself is called the blastula 
or blastoderm. This stage also is common to all the many- 
celled animals. The next stage in embryonic development 
is formed by the bending inward of a part of the blasto- 
derm cell layer, as shown in Fig. 39,9. This bending in 
may produce a small depression or groove; but whatever the 
shape or extent of the sunken-in part of the blastoderm, it 
results in distinguishing the blastoderm layer into two 
parts, a sunken-in portion called the endoblast and the 
other unmodified portion called the ectoblast. Hndo- means 
“within,” and the cells of the endoblast often push so far 
into the original blastoderm cavity as to come into contact 
with the cells of the ectoblast and thus obliterate this cavity 
(Fig. 39, 2). This third well-marked stage in the embry- 
onic development is called the gastrula* stage, and it also 
* This gastrula stage is not always formed by a bending in or in- 
vagination of the blastoderm, but in some animals is formed by the 
splitting off or delamination of cells from a definite limited region of 
