206 MR GEORGE BROOK ON THE 



ovum. The immediate effect of fertilisation is that a true germinal disc is 

 formed, of which the protoplasm already existing in the ripe unfertilised ovum 

 forms only a part. Such changes never take place without the stimulating aid 

 of spermatozoa. Their exact nature will be described in due course ; for the 

 present it will be sufficient to describe the behaviour of the germinal proto- 

 plasm in the unfertilised egg. Simply stated, the germinal protoplasm acts 

 as so much passive material so long as the egg remains unfertilised. It collects 

 very slowly to the surface, it is true, but in a very different manner to that in 

 which it would if fertilised. There is a gradual accumulation at the surface, 

 and day by day the protoplasmic network is withdrawn more and more from 

 the centre, but it never collects into a disc-like prominence in the unfertilised 

 ovum. After a week's immersion in sea water an unfertilised ovum presents 

 the appearance shown in fig. 5. It only differs from the recently-matured 

 ovum in having more protoplasm around the circumference and less towards 

 the centre of the egg. The protoplasmic filaments are never withdrawn to 

 nearly the same extent in the unfertilised as in the fertilised egg. In the 

 fertilised egg there are a number of branching protoplasmic filaments connecting 

 the germinal disc with the yolk, but in the yolk pole the protoplasm is almost 

 entirely withdrawn. In the unfertilised egg which has been some days in sea 

 water this is not the case. The protoplasm forms a comparatively even layer at 

 the surface of the yolk, and there is no division into animal and vegetative poles, 

 the protoplasmic filaments not being withdrawn more at one part than at another. 

 It should be pointed out that this partial collection of the germinal protoplasm 

 at the surface will take place ivJiether the egg is placed in sea water or not. In 

 order to conduct the experiment already referred to, ripe females were kept for 

 a varying time in moist cloths. Sections of eggs kept under such conditions 

 show that the germinal protoplasm begins to collect at the surface of the yolk 

 soon after the egg is ripe, and that the amount of protoplasm found at the 

 surface is, roughly speaking, proportional to the time which has been allowed 

 to elapse before examination. Thus, then, sea water has nothing to do with 

 causing the protoplasm to collect at the surface, nor, so far as I could make 

 out, is this accomplished more rapidly in sea water than in the ovary itself. 



Formation of the Germinal Mound. 



The fact that at the time of impregnation the ovum of the herring exhibits 

 nothing of the nature of a germinal disc, in the ordinary sense of the word, is a 

 point of very great interest. In the history of the majority of fish ova, the 

 influence of the sperm is not necessary for a separation of the germinal disc 

 from the yolk, as this separation has already taken place before fertilisation. 

 The case of the herring is, therefore, specially interesting from the fact that this 

 accumulation of the germinal protoplasm at one pole can be watched under the 



