58 



FOUNDATIONS OF BIOLOGY 



PRIMARY GERM LAYERS are established which are characteris- 

 tic of the developing animal, and from these the specialized 

 tissues which compose the various systems of organs of the 



I) 



G 



II 



FIG. 19. Early stages in the development of the egg of a Sea Urchin. A-F, cleavage 

 and formation of the blastula; G, section of blastula showing the beginning of gastrula- 

 tion; H-I, early and later gastrula stages, a, ectoderm; b, endoderm; c, blastocoel; 

 d, blastopore, leading into the enteric cavity; e, cells, arising from the endoderm, 

 destined to form the mesoderm. 



adult are derived. For example, the ectoderm by cell divi- 

 sion and differentiation gives rise to the outer skin and central 

 nervous system; the mesoderm to muscular, connective, and 

 supporting tissues and the blood vascular system; while the 



