34 



Appendices to Fourth Annual Report 



unfertilised ovum the germinal protoplasm is distributed throughout the 

 yolk, and it does not mingle with the food material, but forms a network 

 filling up the spaces between the yolk spheres. An examination of 

 sections of the ovum shows that the conclusions arrived at in the experi- 

 mental work are justified, and that the germinal protoplasm does not col- 

 lect at the surface of the yolk until after fertilisation. Kupffer asserts that 

 at the time the egg reaches the sea-water from the oviduct there is no 

 trace of a germinal disc, nor even any separation into formative and 

 nutritive yolk. With the first part of this statement I thoroughly agree. 

 In the ordinary sense of the word there is no germinal disc in the ripe 

 unfertilised ovum. The germinal protoplasm nevertheless exists, but 

 being generally distributed instead of forming a special aggregation at one 

 part, it is not easily made out in the living egg. This has been already 

 pointed out by Hoffmann, though I am inclined to think that he also has 

 not thoroughly understood the formation of the germinal disc in the 

 herring, or rather what I shall term the germinal mound. 



Soon after fertilisation the first change to be noted is the separation of 

 the egg membrane from the yolk. This commences at first in one part 

 only, but quickly extends all around the egg, so that the vitellus comes 

 to lie loosely in an enlarcjed egg capsule. The space between the two is 

 filled with water, in which a portion of the egg contents is dissolved. 

 This space is the perivifelline space, the breathing chamber of Eansome. 

 The increase in diameter which results from the inception of sea- water is 

 about '35 mm. The unfertilised egg averages 1"17 mm. in diameter, and 

 when the breathing chamber is completely formed this diameter is 

 increased to 1'52 mm. 



As the egg membrane leaves the yolk the surface of the latter can be 

 more easily studied. It is then seen that the 'yolk granules' on the 

 surface are rapidly disappearing. The neighbouring granules run together 

 and flatten out into a thin pellicle, which soon becomes indistinguishable 

 on the surface of the larger yolk spheres. The germinal protoplasm now 

 collects to the surface of the yolk. Having failed to recognise the network 

 of germinal protoplasm in the unfertilised egg, Kupffer concluded that 

 the protoplasm of the germinal mound is derived almost solely from the 

 vitellus after fertilisation. He describes the first stage in the process as 

 the formation of a series of clear vacuoles on the surface of the yolk, 

 which have the appearance of small transparent spots. These increase 

 rapidly in size, and are pushed forward towards the centre of the yolk as 

 a coarse network of tubes. With the appearance of the clear vacuoles 

 the germinal protoplasm begins to collect on the surface of the yolk. 

 According to my own observations I am led to reverse the process, and 

 to conclude that the tube-like structures do not grow from the surface 

 downwards, but are present in the unfertilised egg, and are gradually 

 withdrawn into the surface layer. With the act of fertilisation an 

 activity is set up in the germinal protoplasm, which causes it to collect 

 rapidly at the surface. Very early in this process the outer yolk spheres 

 are a little wider apart than those towards the centre, and the protoplasm 

 as it collects widens and fills up the spaces between them A little later 

 the protoplasm is almost entirely Mdthdrawn from the centre of the egg, 

 and there is then a thin layer of germinal protoplasm on the surface, with 

 a number of branched root-like processes extending some distance into 

 the yolk. It is, I think, this collecting protoplasm which Kupffer has 

 mistaken for vacuoles and his series of coarse tubes. To begin with, the 

 protoplasm forms a network throughout the yolk. After fertilisation the 

 protoplasm collects at the surface, and in doing so the more peripheral 

 strands increase in thickness at the expense of those more centrally 



