166 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
form the outer circumferential wall of this hollow sphere (f). This is 
called the blastula 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 develop- 
ment is formed by the bending inward of a part of the blastoderm cell 
layer, as shown in (g) (or the splitting off inwardly of cells from a 
special part of the blastula cell layer). 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 or inner portion called 
Fic. 35.—First stages in the embryonic development of the pond snail, 
Lymnaeus. a, egg cell; 6, first cleavage; c, second cleavage; d, third cleavage; 
e, after numerous cleavages; f, blastula—in section; g, gastrula just forming— 
in section; 4, gastrula completed—in section. (From Jordan and Kellogg, after 
Rabl.) 
the endoblast and the other unmodified portion called the ectoblast. 
Endo- 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 (#). This third 
well-marked stage in the embryonic development is called the gastrula 
stage, and it also occurs in the development of all or nearly all many- 
celled animals. 
In the case.of a few of the simple many-celled animals the embryo 
hatches—that is, issues from the egg at the time of or very soon after 
reaching the gastrula stage. In the higher animals, however, develop- 
ment goes on within the egg or within the body of the mother until 
the embryo becomes a complex body, composed of many various 
