THE CONTINUITY OF LIFE 



231 



C. Hydra. 



ec.en.gc 



Cell Division 

 (Embryological 

 development) 



Zygote (z) 

 produces ani- 

 mal contain- 

 ing germ cells 

 (g.c.) and two 

 layers of spe- 

 cialized somatic 



cells, the 

 ectoderm (ec.) 

 and endoderm 

 (en.). 



Budding 



(Asexual re- 

 production) 

 Part of animal 

 separates from 



parent and 

 leads separate 

 existence. 



Cell Division 

 (Gamete for- 

 mation) 

 Certain germ 

 cells produce 

 eggs (e); others 

 produce sperm 

 (sp.). 



Permanent 

 Conjuga- 

 tion 

 (Fertilization) 

 one sperm 

 unites with 

 one egg, form- 

 ing a zygote 

 (z). 



Cell Division 



(Embryological 



development) 



Zygote (z) 

 produces ani- 

 mal, etc. 



D. Earthworm. 



<& 



Cell Division 

 (Embryological 

 development) 

 Zygote (z) pro- 

 duces animal 

 containing germ 

 cells (g.c.) and 

 three layers of 



specialized 



somatic cells: 



the ectoderm, 



mesoderm, and 



endoderm. 



Cell Division 

 (Gamete forma- 

 tion) 

 Certain germ 

 cells produce 

 eggs (e); others 

 produce sperm 

 (sp.). 



Permanent 



Conjugation 



(Fertilization) 



One sperm unites 



with one egg, 

 forming a zygote 

 (z). 



Cell Division 

 (Embryological 

 development) 

 Zygote (z) pro- 

 duces animal, 

 etc. 



Fig. 154. — Diagrams to illustrate the general reproductive cell cycle in 

 A, a unicellular organism {Paramecium); B, a colony of cells (Volvox); C, a 

 simple Metazoon (Hydra); and D, a more complex Metazoon (Earthworm). 

 (Modified, from Hegner.) 



