204 MR GEORGE BROOK ON THE 



not bring out any differentiation in the germinal protoplasm. The latest phase of 

 the germinal vesicle which I have observed is represented in fig. 4. 



It is here somewhat quadrangular, but irregular in outline, and from its 

 margin fine threads of protoplasm are seen to penetrate the yolk mass. With a 

 magnifying power of 700 diameters a beautiful reticular arrangement of the 

 fibrillar can be made out. There are distinct thickenings at the nodes, and 

 besides the fibrillar reticulum there are a very large number of very small 

 granules, which stain deeply with carmine. I have not been able to make 

 out the process of degeneration which the germinal vesicle probably under- 

 goes prior to the ovum becoming mature and ready for fertilisation. 



The difference then between the ripe ovum of the herring and that of other 

 Teleostean fishes appears to rest in the fact, that at the time the herring ovum is 

 ready for impregnation the germinal protoplasm is not collected into a definite 

 layer. - Changes in the arrangement of the egg contents therefore occur in the 

 herring ovum after fertilisation, which in the majority of fish ova have already 

 been brought about without the aid of spermatozoa. 



Kupffer also maintains that the relationship of the various parts of the ripe 

 unfertilised ovum remains unaltered in sea water, so long as the egg remains 

 unfertilised; in other words, that sea water, per se, has no influence on the 

 ovum. On this point Boeck and others are at variance with Kupffer. The 

 experiments which I have already described* were in part devised to test this 

 point, and all tend to show that Kupffer's conclusion was justified. Others 

 which were conducted more recently only tend to strengthen the same view. 

 In the first place, the egg membrane does not separate from the yolk unless it 

 has previously been penetrated by spermatozoa. Whether these enter at the 

 micropyle only, or may force their way through the egg envelope at any part, 

 I have not been able to decide; but the fact remains that unless an egg is sub- 

 mitted to the action of spermatozoa, the egg membrane does not separate to 

 any appreciable degree from the yolk. In this connection reference may be 

 made to my experimental researches of the fertilisation of herring ova (7). 

 In one experiment, eggs which had remained unchanged for tiventy-four hours in 

 sea water were submitted to the action of spermatozoa. Within an hour the egg 

 membrane had separated to a considerable extent from the yolk in a large 

 number of the eggs. Although in most cases the eggs at this time retained only 

 a feeble vitality, and the viscous covering of the egg membrane had become so 

 hardened as to offer considerable obstruction to the entrance of spermatozoa, 

 a breathing chamber was formed in the normal manner. It is evident, then, 

 that the entrance of spermatozoa is necessary before this separation can take 

 place. If the micropyle is an open canal, communicating freely with the 

 surface of the yolk, there appears no reason why water should not enter the 



* Annual Keport of the Fishery Board for Scotland, 1885. 



