Relation of Yolk to Blastoderm in Teleostean Fish Ova. 189 



equally well for the Gadiclce, and indeed for all pelagic fish 

 ova which I have examined. In the herring and like 

 forms a similar arrangement is found, only that the com- 

 munication between protoplasm and yolk is never so com- 

 pletely lost as in the pelagic ova. In the Salmonidse, both 

 before segmentation commences and throughout this process, 

 a comparatively thick cortical layer of protoplasm is found, 

 which is intimately connected with the yolk. Here the 

 cortical protoplasm has an additional function. The yolk 

 substance coagulates instantly when brought in contact with 

 water. The cortical protoplasm acts as a protective mem- 

 brane and prevents this coagulation. Oellacher drew atten- 

 tion to this point fourteen years ago, and also showed that 

 this cortical layer must be regarded as a part of the germinal 

 protoplasm of the ovum. 



Let us now reflect on the structure of a Teleostean ovum 

 immediately prior to the commencement of the segmentation 

 process. Oellacher, in the paper referred to, uses what ap- 

 pears to me a very good simile. He compares the mero- 

 blastic ovum of the trout to a gigantic fat-cell, the extension 

 of the cortical protoplasm around the yolk being regarded as 

 equivalent to the thin extension of the cell - protoplasm 

 around the fatty substance of the latter. There is another 

 point of similarity which is important. The fat contained 

 within the protoplasm of the fat-cell is a store of passive 

 food material, which can be drawn on as occasion requires. 

 Precisely the same relation exists between germinal proto- 

 plasm and food-yolk in the fish ovum, and doubtless also in 

 all meroblastic ova. The distinguishing feature of a mero- 

 blastic ovum is that its constituent parts are practically 

 separated into two distinct areas — protoplasm and yolk. 

 The yolk is included within the cell substance, but does not 

 form an essential part of it, so far as its characters as a cell 

 are concerned. In the meroblastic ova of many invertebrates 

 — for instance, in Feneus — the yolk takes a more or less 

 central position, while in the fish ovum the yolk, although 

 situated eccentrically, is still completely surrounded by the 

 cell-protoplasm. The difference between centrolecithal and 

 telolecithal ova thus becomes more one of degree than of kind. 



