166 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



form the outer circumferential wall of this hollow sphere (/). This is 

 called the Uastula 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 



FIG. 35. First stages in the embryonic development of the pond snail, 

 Lymnaeus. a, egg cell; b, first cleavage; c, second cleavage; d, third cleavage; 

 e, after numerous cleavages; /, blastula in section; g, gastrula just forming 

 in section; h, gastrula completed in section. (From Jordan and Kellogg, after 

 Rail.) 



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 (h). 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 



