THE FERTILIZATION-PROCESS 



This brief account of the history of the germ-cells sub- 

 sequent to their differentiation from the somatic cells 

 teaches us that they pursue diflFerent courses. Without 

 going into details, I call attention further to three facts of 

 general significance for all that follows in my presentation 

 of the problem of fertilization. The first is that there does 

 not exist a single animal whose spermatozoa do not originate 

 from an egg; the second, that no animal spermatozoon ever 

 develops alone; and the third, that fertilization is not indis- 

 pensable for the development of animal eggs, for, as we shall 

 see in the chapter on parthenogenesis, many eggs develop 

 normally or experimentally without the presence of sper- 

 matozoa. These three facts obviously warrant the con- 

 clusion that the egg carries the greater burden in fertilization. 

 That the spermatozoon is itself derived from the egg is a 

 fact to be emphasized. Even where it is indispensable for 

 the egg's development, we can not in the light of its origin 

 regard it as the perfect antipode of the egg. Whatever its 

 highly specialized powers and functions, they rest upon 

 derivatives of the egg-substance whence the spermatozoon 

 came. 7'hat the spermatozoon never develops without the 

 egg, whilst the egg can develop without the spermatozoon 

 is another way of saying that the egg alone carries the bur- 

 den of development. In these differences between the 

 gametes we recognize where lie the powers not only of 

 fertilization, but also of the whole course of the future 

 development; the spermatozoon is a cell reduced to the 

 minimum potency through its differentiation, the Qgg is of 

 all cells known the most potent by virtue of its peculiar 

 differentiation. Structurally, the great difference between 

 spermatozoon and egg relates to the cytoplasm. Out of 

 the egg's cytoplasm the future adult organism is differ- 

 entiated. I propose in the following pages to prove that 

 fertilization, the initial act in the differentiation of the egg, 

 is likewise a cytoplasmic phenomenon. In order to reach 



149 



