CHAPTER XIX 

 FERTILIZATION 



The entire organism may be compared to a web of which the warp 

 is derived from the female and the woof from the male. — Huxley. 



Now that we are familiar with the method of gamete formation 

 and its contribution to the continuity of life, it is in order to con- 

 sider some important details of the structure of the gametes them- 

 selves, and the significance of the complex series of phenomena 

 that they initiate at fertilization. The biological importance of 

 fertilization and the part it plays in the life of the individual or- 

 ganism and the race has aroused the interest of philosophers and 

 scientists since the time of Aristotle, but it is only within the past 

 half-century that at least a partial answer has been forthcoming 

 from a critical analysis of the gametes and their product, the 

 zygote. 



A. Gametes 



The gametes, while exhibiting in certain cases peculiar adapta- 

 tions to special conditions, are remarkably similar in general struc- 

 ture throughout the animal series. It is possible to arrange a 

 series of lower forms which shows various stages in sex differen- 

 tiation. Beginning with those in which both gametes are struc- 

 turally similar, we pass by slow gradations to others in which the 

 egg is a relatively large, passive, food-laden cell and the sperm a 

 minute, active, flagellated cell. 



As a matter of fact, the egg is subject to somewhat more varia- 

 tion in size and general appearance than the sperm, for after 

 fertilization it must be adapted to meet the special conditions of 

 development peculiar to the species. Thus, for instance, the actual 

 size of the egg in animals is determined chiefly by whether the de- 

 veloping embryo is in the main dependent upon food stored in the 

 cytoplasm of the egg itself, or upon some outside source, such as 

 the sea water in which it floats, or the tissues of the parent. The 

 first case is well illustrated by a Bird's egg in which the so-called 

 TiOLK is the egg cell proper, hugely distended by stored food, and 



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