of the Fishery Board for Scotland. 



35 



situated. The branching root-like processes are therefore not pushed 

 down from the surface, but are part of the germinal protoplasm, which 

 has not yet been included in the surface layer. In sections of the egg at 

 this stage, w^hich have been mounted unstained, the germinal protoplasm 

 is very transparent, whereas the yolk spheres are quite granular, and of a 

 yellowish tint. This accounts for the tubular appearance of the channels 

 seen in the living egg. 



In the unfertilised ovum the protoplasm always collects more or less 

 at the surface after a time, but its behaviour is quite different to that in 

 the fertilised ovum. So long as an ovum remains unfertilised the 

 germinal protoplasm acts as so much passive matter. It collects slowly 

 at the surface, and day by day the protoplasmic network is withdrawn 

 more and more from the centre. The collection goes on equally all round, 

 and the protoplasm does not appear to be withdrawn to a greater extent 

 in one part than in another. After a week's immersion in salt water a 

 section of an unfertilised egg only differs from that shown in fig. 19, in 

 having more protoplasm at the surface and less in the centre. The proto- 

 plasmic filaments are, however, never so completely withdrawn in the 

 unfertilised as in the fertilised egg. It should be pointed out that this 

 partial collection of the germinal protoplasm at the surface takes place 

 whether the egg is placed in sea water or 7iot. In order to conduct the 

 experiments already referred to (p. 31) ripe females were kept for a 

 varying time in moist cloths, and eggs were pressed from the genital 

 opening from time to time as they were required. Sections of these eggs 

 showed that the germinal protoplasm begins to collect at the surface of 

 the yolk in a passive manner soon after the egg is ripe, and that the 

 amount of protoplasm found at the surface was, roughly speaking, pro- 

 portional to the time which had been allowed to elapse before examina- 

 tion. Thus, then, sea water has nothing to do with causing the protoplasm 

 to collect at the surface, nor, so far as I could see, is this accomplished 

 any more rapidly in water than in the ovary itself. 



In a batch of eggs which have been fertilised artificially there are 

 always a few which have escaped fertilisation. These can readily be 

 distinguished with the naked eye, partly on account of their greater 

 opacity, and partly because they retain their original size, there having 

 been no inception of water. Such eggs, up to the end of the incubation 

 of the batch, always have the germinal protoplasm more or less distri- 

 buted throughout the yolk. 



Let us return now to a consideration of the behaviour of the fertilised 

 ovum. 



At first the protoplasm, as it collects, forms a comparatively even layer 

 around the yolk. Soon, however, it assumes a digestive function, and large 

 masses of yolk are incorporated within its substance and assimilatsd. 

 During this period the protoplasm exhibits slow undulating movements 

 around the yolk. The protoplasm increases considerably in bulk at thft 

 expense of the yolk. After a time the greater part of the protoplasm 

 collects at the germinal pole, and there forms the germinal mound. 

 Sections at this stage show that the whole of the protoplasm has not yet 

 been withdrawn from the interior. From the base of the germinal mound, 

 which is not weU defined, a number of branching filament press down for 

 a considerable distance between the yolk spheres, and there is always a 

 cortical film of protoplasm around the yolk (fig. 22). 



There is frequently a collection of protoplasm at the pole opposite to 

 the germinal mound. It appears that as the protoplasm is preparing to 

 collect at the germinal pole, it oscillates between the two opposite poles 



