THE MEANING OF EVOLUTION 



255 



-Coelerfjeron 



c 



structure in the early developmental stages of very 

 many animals belonging to widely separated groups. 

 In all of these the ovum segments into a mass of cells, 

 which then become arranged as a hollow ball (A). One 

 side of this ball becomes pushed in so that the inner 

 part of the hollow sphere becomes opposed to the inner 

 wall of the upper part. Thus a little sac, consisting 

 of two layers of cells, ectoderm 

 and endoderm, and opening to 

 the outside by an aperture, the 

 blast opore, is formed (B) . This 

 is essentially the anatomy of 

 the schematic Ccelenterate 

 animal Hydra, for instance, 

 strongly suggests it. Suppose 

 now that the lips of the blas- 

 topore fuse together at one 

 place so that there are two 

 openings into the cavity of 

 the gastrula instead of one ; 

 and suppose that the spherical D 

 organism elongates so as to 

 form a cylinder, the elongation 

 involving the fused part of 

 the blastoporic region. Then 



we obviously have a worm-like animal with an alimen- 

 tary canal, a mouth and an anus (C). Suppose further 

 that an additional layer of cells becomes formed between 

 the endoderm and ectoderm by proliferation from one 

 of these tissues, and suppose that this becomes double 

 and that a cavity appears between the two sheets 

 of cells forming this middle layer : this cavity be- 

 comes the body cavity or ccelom (D). Now such bias- 

 tula and gastrula stages appear in the ontogeny of 

 animals belonging to widely different groups, and 



FIG. 26. 



