152 



@-p-g' 



GENERAL ZOOLOGY 



X 



H / J 



FIG. 64. Scheme for the developmental stages of a metazoan. A, B, mat- 

 uration of egg and sperm cells a, first division; 6, two cells resulting from 

 division; c, cells after next division; d, ripe egg or sperm cell. C, zygote formed 

 by union of ripe egg and sperm, now dividing. D, cleavage stages. E, section 

 through blastula stage the stippled cells represent the portion to become ento- 

 derm. F, section during gastrula formation; x, cleavage cavity. G, section 

 through mature gastrula; ec. ectoderm; en, entoderm; y, archenteron, or primitive 

 digestive cavity. H, young metazoan with tubular digestive system. H-K, 

 stages in the formation of the coelom or body cavity; the lower figure of each 

 stage represents a cross section of the one above; co, coelom. 



