ORIGIN OF THE INDIVIDUAL 



215 



C. Hydra. 



D. Earthworm 



g.c. 



CELL DIVISION CELL DIVISION 

 (Embryological (Gamete forma- 

 tion) 



Certain germ 



cells produce one 



egg (e) and 



polar bodies 



(pb.) ; others 



produce many 



PERMANENT CELL DIVISION 



CONJUGATION (Embryological 



(Fertilization) development) 



One sperm unites Zygote (z) pro- 



with one egg, duces animal, 



forming a zygote etc. 

 (z). 



development) 

 Zygote (z) pro- 

 duces animal 

 containing germ 

 cells (g.c.) and 

 three layers of 



specialized 

 somatic 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) ; (B) a colony of cells ( Volvox) ; (C) a simple Metazoon 

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



