ORIGIN OF THE INDIVIDUAL 



215 



C. Hydra. 



ec.en.g.c. 



PERMANENT CELL DIVISION 



CONJUGATION (Embryological 



(Fertilization) development) 



One sperm unites Zygote (z) pro- 



with one egg, duces animal, 



forming a zygote etc. 

 (z). 



CELL DIVISION CELL DIVISION 



(Embryological (Gamete forma- 



development) tion) 



Zygote (z) pro- Certain germ 



duces animal cells produce one 



containing germ egg (e) and 



cells (g.c.) and polar bodies 



three layers of (pb.); others 



specialized produce many 



sojnatic cells, sperm (sp.). 

 the ectoderm 

 (ec.), mesoderm 

 (ms.), and endo- 

 derm (en.). 



FIG. 115. Diagrams to illustrate the general reproductive cell cycle in (A) a uni- 

 cellular organism (Paramecium) ; (E) a colony of cells (Volvox); (C) a simple Metazoon 

 (Hydra); and (D) a more complex Metazoon (Earthworm). (From Hegner.) 



