166 A. H. EEG1NALD BULLER. 



often stop in tliuir rotation^ and that the head becomes fixed 

 in the jelly. Sometimes the spermatozoon then succeeds in 

 boi-ing its way through, and may then reach the living 

 protoplasm. In most cases, however, the head of a sper- 

 matozoon which has rotated a number of times gets stuck 

 in the outer layer of the jelly, and no successful penetration 

 occurs. 



The gelatinous coat of an egg which has only been in 

 water a few minutes is much more difficult for the spermatozoa 

 to penetrate than that of an egg which has been in water 

 several hours. Comparative experiments easily demonstrated 

 this point, the difference being really striking. The jelly, as 

 already mentioned, swells in water, and gradually nearly 

 doubles its original breadth. At the same time it becomes 

 softer. When spermatozoa are added after the swelling has 

 taken place, scarcely a single spermatozoon is seen to I'otate 

 upon the eggs ; on the contrary, they nearly all succeed 

 in fixing their heads in the jelly, and the majority penetrate 

 almost up to the living egg. 



Dewitz^ believes that the rotation of the spermatozoa of 

 Blatta upon surfaces is of prime importance in enabling the 

 spermatozoa to find their way into themicropylesof the eggs. 

 'Jliis may well be the fact. In the case of the Echinoidea, 

 however, there are no micropyles, and the gelatinous coat is 

 everywhere penetrable. Further, rotation upon the eggs 

 appears not to be the rule. It seems to nie, there- 

 fore, that the fact that the spermatozoa Avill rotate upon 

 resistant surfaces has no special biological significance in 

 respect to fertilisation. On the other hand, the ability of the 

 spermatozoa to cling to surfaces and to get stuck to them by 

 the ])oiuted end of their heads is of great importance in 

 causing them not to leave the gelatinous coat of an L'gg after 

 having come in contact with it, and in penetrating the same. 



' Dewitz, loc. cit. 



