THE BIOLOGY OF THE CELL SURFACE 



sperm-nucleus once in this egg lies inert whilst the egg- 

 nucleus alone takes part in development, is a pretty state- 

 ment without much scientific value. If an Qgg can not 

 develop without fertilization, the criterion of its having 

 been fertilized is its development. If the spermatozoon 

 is essential to the initiation of development, fertilization has 

 been efTected, if this egg develops. To speak of this case 

 and of experimental conditions in which the egg-nucleus 

 though present is inhibited from taking part in cleavage as 

 partial fertilization is to ignore the essential problem. If 

 an egg-fragment without a nucleus develops after having 

 been entered by a spermatozoon, it has been fertilized. To 

 retain the old definition of fertilization as the union of egg- 

 and sperm-nucleus is to violate both fact and logic. 



But whatever the situation with respect to a union of the 

 egg- and sperm-nuclei, always a division-spindle arises and 

 in this the fertilization-process reaches its culmination. At 

 the poles of this spindle are star-like formations, the asters. 

 These beautiful formations are especially striking in many 

 living eggs and since they are always present in the nuclear 

 division of cleaving fertilized eggs, they have been held to be 

 the cause of fertilization. Thus Boveri postulated a theory 

 of fertilization which, though he himself later abandoned it, 

 is even to-day defended by man}' writers: namel}', that the 

 essential feature in fertilization is the introduction into the 

 tg^ of two centrosomes by the spermatozoon, about which 

 two asters form which persist as the asters of the egg's divi- 

 sion-spindle. Were it not for the fact that many writers 

 still uphold this definition, we could dismiss it at once inas- 

 much as we have seen in the descriptions of the fertilization- 

 process of the four types of eggs given above that the cleav- 

 age-centres can not be always shown to have arisen from 

 sperm-centres. Moreover, whilst in the majority of eggs the 

 cleavage-centres probably are continuations of both sperm- 

 centres, there are eggs in which the centres come one from 

 the egg-nucleus and one from the sperm-nucleus, as in eggs 



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