252 



ANIMAL BIOLOGY 



surrounded by nutritive and protective envelopes consisting of 

 the 'white of the egg,' shell membranes, and shell which are formed 

 by secretion from the walls of the oviduct during the passage of 

 the egg to the exterior. On the other hand, the eggs of Mammals, 

 for instance of the Rabbit and Man, are very small — the human 

 egg being less than 1/1 25th of an inch in diameter — since their 



Granular 'polar* cytoplasm 



First polocyte 



Second polocyte 

 spindle 



acuolated ectoplasm 



Dense endoplasm 



Vacuolated 

 endoplasm 



Inner membrane 

 Outer membrane 



Fig. 168. — A, section through the egg of a primitive Vertebrate, the Lam- 

 prey. B, sperm of the same species, drawn to scale. (From Kellicott, after 

 Herfort.) 



essentially parasitic method of development in the uterus renders 

 superfluous the storage of any considerable amount of food ma- 

 terial in the egg cytoplasm. (Figs. 7, 133, 168, 177.) 



With the specialization of the egg along lines which render it 

 non-motile, it has devolved upon the sperm to assume the func- 

 tion of seeking out the egg for fertilization. It does this in most 

 cases by active lashing of its flagellum. This necessitates a fluid 

 medium in which the sperm can swim, and such is provided by the 

 environment in which the organism lives or, in the case of most 

 higher animals, where fertilization takes place within the oviduct, 

 by special fluids secreted for the purpose. 



A question of much interest is how the actual meeting of the 



