ii4 FERTILISATION [CH. 



later spermatozoa. It has also been maintained that the 

 fluid between the egg and the membrane has an agglutin- 

 ative effect upon spermatozoa, and inhibits the movement 

 of any which may have penetrated within the membrane. 



In a number of animals only the head and middle-piece 

 of the spermatozoon enter the egg; in others the whole 

 spermatozoon penetrates into it, but the tail soon breaks 

 off. The head and middle-piece then rotate, so that the 

 middle-piece precedes the head as they sink deeper into the 

 egg substance. Round the middle-piece radiations then 

 begin to appear in the egg-protoplasm, and the middle-piece 

 is either converted into the centrosome which will give rise 

 to the spindle of the first segmentation division, or, accord- 

 ing to some investigators, the centrosome arises from the 

 interaction of the middle-piece and the egg cytoplasm (PL 

 XII, B-I}. 



From this stage onwarc} the details of the process of 

 fertilisation differ somewhat according to whether the egg 

 is one of those which undergoes its maturation-divisions 

 before or after the entrance of the spermatozoon. If the 

 maturation-divisions have already taken place before the 

 spermatozoon enters, as for example in Echinoids, the egg- 

 nucleus is ready immediately for fertilisation. In such eggs 

 the sperm head, preceded by the middle-piece or developing 

 centrosome, sinks rapidly into the egg, at first towards the 

 centre, and then, if the egg-nucleus is excentric, towards the 

 egg-nucleus ; as the sperm head approaches, the egg-nucleus 

 also begins to move to meet it. Especially in yolky eggs, 

 the sperm head, as it sinks into the egg, leaves behind it a 

 definite track in the egg substance, so that the course which 

 it has followed remains visible. 



The nature of the attraction between the two nuclei is 

 quite unknown. As it sinks in, the sperm head begins to 



