THE FERTILIZATION-PROCESS 



of many snails. In addition, there are eggs in which the 

 asters of the division-spindle arise entirely around the egg- 

 nucleus, the sperm-nucleus never showing any trace of 

 astral radiations, as in eggs of trematodes. Finally, as we 

 have seen, the division-spindle in eggs of sea-urchins shows 

 asters devoid of centrosomes. What in these Boveri iden- 

 tified as centrosomes have been proved beyond doubt to 

 be bodies located in the middle-piece of the spermatozoon 

 that are cast off into the egg-cytoplasm and have no causal 

 relation to the asters.^ For other eggs it has also been 

 shown that granules in the middle-piece of the sperma- 

 tozoon can not be identified as centrosomes.- In one egg 

 only, that of Nereis, has it been shown that the centrosomes 

 arise out of the sperm-nucleus. 



The conclusion is patent: as it is the case with the stage 

 in maturation when eggs reach the fertilizable condition, 

 and with the mode of union of the egg- and sperm-nuclei, 

 so with the origin of the cleavage-centres — all possible varia- 

 tions exist. Hence we find no constancy with respect to 

 the origin of the cleavage-centres and therefore can not 

 define fertilization as the importation into the egg of cen- 

 trosomes by the spermatozoon. 



Failing to discover in any of the events that happen dur- 

 ing the internal phase of the fertilization-process the funda- 

 mental act in fertilization, we turn to the external phase, 

 to the changes at the egg-surface incident to the attach- 

 ment of the spermatozoon. All animal eggs in response 

 to sperm-attachment show some visible change in the ecto- 

 plasm. In the following chapter I present my hypothesis 

 that underlying these changes at the egg-surface is the 

 fundamental act in the fertilization-process. I denominate 

 this act the fertilization-reaction. 



^ See Meves, 191 2; Just, 192'ja. 



^ According to Meves there are not Vivo hut jive such granules in the 

 sperm-middle-piece of Mytilus. 



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