THE FER T I LIZA TION-PROCESS 



cleavage-nucleus, about which the first mitotic figure arises. 

 Thus the fertilization-process in these four examples resolves 

 itself into two phases — an external, that concerns the ecto- 

 plasm, and an internal, that concerns the nuclei. 



Fertilization means in the strict sense of the word the 

 coming together of egg and spermatozoon, for without it 

 there is no fertilization. Hence obviously, the initial act 

 in the whole chain of events is the contact between the 

 two partners. If we endeavor to state the definite result 

 of these events, we can say that it is the development of 

 the egg. But it would certainly be impractical to define 

 fertilization as some have defined it, namely, that it is 

 complete only when the developed organism has reached 

 that stage in its development when in its germ-cells the 

 chromosomes pair. Since in all animal eggs a mitotic figure 

 is established for the first division-nucleus, it is far more 

 practicable to take this feature, common to all eggs after 

 development has begun, as the end-point of the series of 

 events that begins with the coming together of egg and 

 spermatozoon. 



These events we speak of as the fertilization-process; 

 this resolves itself, as said above, into an external and an 

 internal phase. I shall now endeavor to determine which 

 of these must be regarded as the fundamental phase in fer- 

 tilization, that is, during which takes place the funda- 

 mental happening, event, in fertilization. I first discuss 

 the internal phase of the fertilization-process, since it 

 involves more generally known phenomena and since pre- 

 vailing opinion among biologists places the basic event in 

 this phase. I begin with the question of the union of the 

 egg- and sperm-nuclei. 



Were we to conclude from the descriptions of fertiliza- 

 tion of eggs of the four classes as given above, we could say 

 that fertilization is the union of the egg- and sperm-nuclei. 

 However, cases exist in which egg- and sperm-nuclei norm- 



179 



