FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 221 



each end of the furrow becomes forked. The forked extremities then grow 

 round, so that the two on each side meet. Two cells are thus formed, whose 

 outline is very distinct in the region of the original longitudinal furrow, but gets 

 more and more indistinct towards the periphery of the disc. The two cells thus 

 formed do not enclose the ivhole of the protoplasm in the germinal area. The 

 protoplasm at this stage has a yellowish tinge, and in the living egg the faint 

 yellow shade can be seen to extend outside the limits of the two segmentation 

 spheres (see figs. 1-4, loc. cit.). By another vertical furrow at right angles to 

 the first the germinal disc becomes divided into four cells. It will be remem- 

 bered that in the herring the first equatorial furrow simply completes the base 

 of the four existing segmentation spheres, and that with the completion of this 

 furrow there is a division into archiblast and parablast. In pelagic fish ova, 

 however, the germinal disc rests directly on the yolk sphere, there being no 

 intermediate area which is actively engaged in assimilation at this stage. Thus 

 the base of the early segmentation spheres is not easily recognised. Only in 

 that portion of the germinal disc which rests on the cortical protoplasm, and 

 not on the yolk, could any such furrow be made out. There is, however, reason 

 to suppose that the base of the first two segmentation spheres is not defined, 

 and that even the lateral line marking their periphery does not reach the yolk at 

 this stage, from the fact that the two cells increase considerably in size after 

 their outline has been defined. At the completion of the 4-cell stage the 

 segmented disc is undoubtedly divided off from the cortical protoplasm, and 

 there is a division into archiblast and parablast, as in the herring ovum. 

 Whether the furrow (or equivalent of a furrow) which brought about this 

 separation was formed before or after the second meridianal furrow, I cannot 

 say definitely at present. Whichever be the case, it is clear that this furrow is 

 homologous with the third furrow in the herring ovum, and also with the third 

 in the amphibian ovum. In the case of the Salmonidw, it appears to me also 

 that the point which in reality corresponds with the first equatorial furrow in 

 the amphibian ovum is reached at the 4-cell stage. When the base of these 

 four cells comes to be defined there is still left a cortical layer of protoplasm, 

 and a small quantity also mixed up with the yolk under the disc, as is the case 

 in the herring ovum. 



From what has been already said, it will be seen that I conceive the term 

 Archiblast to be applicable to that portion of the ovum which is usually spoken 

 of as the germinal disc — that portion, namely, which is included in the early 

 segmentation stages. It is for this reason that I have preferred to use the 

 term germinal area in describing all changes in the herring ovum prior to the 

 completion of the 4:-cell stage. The later stages of the segmentation process do 

 not present any features of special interest. A stage in which the germinal 

 disc consists of three rows of cells is shown in fig. 10. About 26 hours after 



