FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 207 



microscope in the living egg. Kupffer has made an elaborate series of inves- 

 tigations on the formation of the blastoderm in the herring ovum. He con- 

 cludes that the germinal disc is formed by the combined influence of sea water 

 and spermatozoa. Kupffer's conclusion is undoubtedly true to a certain 

 extent, though not exactly in the manner in which he intended it. He was of 

 opinion that the act of fertilisation brought about changes in the egg contents 

 which resulted in the separation of the germinal protoplasm from the yolk, and 

 the collection of this into a definite mound at the animal pole of the ovum. 

 Although he mentions a certain small quantity of protoplasm mixed amongst 

 the yolk spheres, he does not appear to have been aware that there is in the 

 ripe unfertilised herring ovum a considerable collection of germinal protoplasm 

 distributed throughout the yolk, the greater portion of which ultimately forms 

 the germinal disc. Nevertheless, he was correct in the sense that the germinal 

 protoplasm is not collected into the form of a disc until after fertilisation. On 

 this account it appears to me that Hoffmann is not entirely justified in his 

 assertion that in the herring, as in other Teleostean fishes, the germinal layer 

 exists before fertilisation. As we have seen, the germinal protoplasm which 

 exists in the unfertilised egg must undergo a further development and growth 

 before a germinal layer exists, which can be compared with that existing in 

 most Teleostean fish ova before fertilisation. Van Bambeke (3) has called 

 attention to the same point, and has shown that in Tinea vulgaris, as well as in 

 the herring, the influence of spermatozoa is necessary before a true germinal 

 disc is formed. If the germinal protoplasm existing in the unfertilised ovum 

 simply collected at the surface after fertilisation, and then commenced to seg- 

 ment, it would be another matter. This protoplasm would then be directly 

 comparable with the germinal disc or germinal layer of other fishes. If by the 

 germinal layer is understood an amount of protoplasm which is distributed 

 throughout the yolk, which, as a result of fertilisation, collects at the surface, 

 grows at the expense of the yolk, and after a considerable increase in its bulk 

 begins to segment, then there is a germinal layer in the ripe unfertilised ovum 

 of the herring. But surely technical terms should have a definite and limited 

 meaning, and it is impossible to regard the germinal layer of such eggs, for 

 instance, as those of the Gadidce, as equivalent with the protoplasm which, in 

 the ripe unfertilised ovum of the herring, is distributed throughout the nutritive 

 yolk. 



Let us glance for a moment at the structure of a pelagic Teleostean ovum. 

 The yolk is very transparent, and is not divided into a large number of yolk 

 spheres, but consists only of one large vitelline sphere, which is comparatively 

 homogeneous in structure. There may or may not be a special condensation of 

 the oily contents into definite oil globules. From the fact that the yolk con- 

 sists only of one large yolk sphere the outline is smooth, and there are no 



VOL. XXXIII. PART I. 2 F 



