188 Proceedings of the Royal Physical Society. 



The primitive ova, when they leave the germinal epithelium, 

 are small simple cells, containing a large nucleus, within 

 which is a dense nucleolus which stains deeply. The cell 

 protoplasm is slightly granular, but has at this time no special 

 supply of food material. Later a follicle is formed, envelop- 

 ing each Qgg, which is also derived from the germinal epithe- 

 lium. The egg then gradually increases in size, the follicle 

 forming the medium through which nourishment is received. 

 It is important to note that from this time onwards the egg 

 contains a new element. The cell protoplasm increases 

 steadily in bulk, but at the same time the excess of nutritive 

 material becomes specially collected in the form of food yolk, 

 which is intended for the nourishment of the future embryo. 

 This food yolk is not distributed throughout the cell proto- 

 plasm, but forms separate masses, which contain little or no 

 protoplasm. The relative distribution of protoplasm and 

 yolk in a fish ovum as it approaches maturity varies con- 

 siderably. In the pelagic group of ova the germinal proto- 

 plasm usually forms a comparatively even superficial layer 

 around the single large yolk sphere. In the herring and 

 some other fishes the yolk consists of a large number of 

 slightly refractive spherules, and the germinal protoplasm is 

 distributed as a network between the yolk spheres. In the 

 Salmonidse a relation exists which is somewhat intermediate 

 between these two types. 



Either before or after fertilisation the bulk of the germinal 

 protoplasm collects at one pole of the ^gg, and segmentation 

 takes place in this area. That the whole of the germinal 

 protoplasm is not included within the so-called germinal 

 disc may be seen from an examination of sections of any 

 Teleostean egg with which I am acquainted. I have already 

 described what takes place in Trachinus. In this species 

 the egg belongs to the pelagic group. The germinal proto- 

 plasm consists of a superficial layer around the single large 

 yolk sphere. After fertilisation the protoplasm gradually 

 sinks to the lower pole, forming a mound, but the periphery 

 of this mound gradually thins off around the yolk. Even 

 while segmentation is in progress there is always a thin film 

 of protoplasm around the yolk. The same description answers 



