vm] ENTRANCE OF SPERMATOZOON 113 



almost certainly a surface-tension phenomenon, and can 

 be induced in unfertilised eggs by certain reagents such as 

 are used to bring about artificial parthenogenesis 1 . 



Although it is often said that |ne fertilisation membrane 

 has the function of preventing the entrance of more than 

 one spermatozoon, it does not provide the only means by 

 which polyspermy is prevented^ as is proved by the fact 

 that if a fertilised egg is cut in two, and the membrane 

 destroyed, the halves do not allow the entrance of additional 

 spermatozoa, while these readily enter the halves of an 

 unfertilised egg. This change in the condition of the egg 

 appears to arise from the influence exerted by the centro- 

 some which is introduced by or at least formed in con- 

 nection with the middle-piece of the spermatozoon. As 

 has been described in Chapter IV, around the centrosome 

 an aster develops. When several spermatozoa enter an egg 

 simultaneously, as may happen when the sperm is concen- 

 trated, or if the egg has been treated with an anaesthetic, 

 an aster develops in connection with each, and these 

 asters~appear to repel one another so as to divide the proto- 

 plasm of the egg among them, without any overlapping. 

 Each centrosome and aster, in- fact, seems to take possession 

 of the area of protoplasm around it, and make it impene- 

 trable either to another spermatozoon or to the rays of. 

 another aster. In an ordinary monospermic egg the same 

 thing happens; the single centrosome "takes possession" 

 of the whole egg and it then becomes impenetrable to any 



1 Some investigators maintain that the membrane is not formed at ferti- 

 lisation, but that a pre-existing membrane is separated from the egg by a 

 "wave of contraction " accompanying the expulsion of waste substances from 

 the egg consequent on the stimulus and resulting change of permeability 

 induced by the entrance of the spermatozoon. The membrane after ferti- 

 lisation, however, must have a different consistency, for it is then impermeable 

 to spermatozoa. For a discussion of this subject see BRACKET (1917). 



D.c. 8 



