HO ELEMENTARY ZOOLOGY. 



A. Not involving a union or fertilization (non- sexual) . 

 Division into equal parts: 



Two parts (cell division, Fig. 14; paramecium, 



some aquatic worms, Fig. 58) . 



Numerous parts (as in Sporozoa, Fig. 18). 

 Division into unequal parts ; budding (as in Hydra, 

 Fig. 46). 



B. Involving a union of cells fertilization (sexual). 



Uniting cells much alike (some protozoa). 



Uniting cells different (ova and sperm: most 



animals) . 



Male and female cells produced by the same 

 individual hermaphroditism. (Earthworm, 

 tapeworm, many snails.) 

 Male and female cells produced by male and 

 female individuals sexual dimorphism. Most 

 higher animals. 



(In several groups both sexual and non-sexual 



methods of reproduction may exist side by 



side or may regularly alternate.) 



4. Fertilization consists in the mingling of the contents 

 of two specialized reproductive cells. The result is an 

 embryo with the power of developing into the mature 

 individual of the species. 



5. The first step of development is cell division. The 

 cells remain together and form a morula of undifferentiated 

 cells. These cells arrange themselves into a two-layered 

 sac (gastrula) . The shape of the gastrula and the rela- 

 tions of these layers to each other and to the cavity, as well 

 as the special mode of development, are determined by the 

 amount and arrangement of the food in the egg. 



6. The later steps consist of the development, from 



