FROM ONE CELL TO MANY CELLS 



73 



in 



O, 







o 



and simple metazoa as representative of the course of evolution of the 

 metazoa because it finds a parallel in the development of the individual 

 among the metazoa. Whether this 

 parallel has any particular significance, 

 or is of interest only as part of the 

 historical development of evolution 

 theory, is uncertain, but the comparison 

 is interesting. 



Individual development begins with 

 a single cell, the egg, which is com- 

 parable to the supposed protozoan 

 ancestor of the metazoa. This egg 

 divides repeatedly (Fig. 61//-/F) to 

 form a group of cells, which may be 

 likened to the protozoan colony. As 

 the division of the egg proceeds farther, 

 it yields a hollow ball of cells, the 

 hlastula (V, VI), which has a form very 

 much like that of Pleodorina and 

 Volvox. It will be recalled that in 

 these organisms the cells are all near 

 the surface, no cells being at the middle 

 of the jelly. The next step in develop- 

 ment is the indentation of one side of the 

 blastula to form a two-layered embryo, 

 the gastrula (Fig. 62A, B). When a 

 diagram of Hydra is placed beside 

 a diagram of a gastrula (B and C, 

 Fig. 62), they are seen to be built 



on the same general plan — that of a two-layered sac open to the exterior 

 at one end. At a stage quite as early as these, some animals show the 



Fiu. 61. — Early metazoan develop- 

 ment. I, undivided egg; II-IV, 

 successive segmentation stages; V, 

 blastula, exterior view; VI, blastula in 

 section to show hollow interior or 

 blastocoele. (From Wilder, ''History 

 of the Human Body," Henry Holt and 

 Company, Inc.) 



ec 



en 



ar 



-<i-bp 



Fig. 62. — Gastrula compared with Hydra. A, beginning of gastrula formation; B, 

 completed gastrula; C, diagram of Hydra; ap, animal pole; ar, archenteron; b, blastocoele; 

 bp, blastopore; ec, ectoderm; en, endoderm; g, gastrovascular cavity; m, mouth. 



distinction between germ and somatic cells (Fig. 63), just as Pleodorina, 

 Volvox, and Hydra do. The germ cells are usually larger than the somatic 



