714 PROFESSOK W. C. M'INTOSH AND MR E. E. PRINCE ON 



Elasmobranchs, and somewhat inconsistently maintains that in Teleosts the origin of the 

 cavity is directly due to cleavage ; whereas, on phylogenetic grounds, it must arise in 

 connection with the peripheral invagination and the formation of the blastopore. If, as 

 Ryder holds (No. 141, p. 492), the Teleostean germ is equivalent to the whole ovum of 

 Rana, then we must look for a segmentation-cavity deeply placed in the former blasto- 

 derm, a fact which Bambeke, as we have seen, considers established for Leuciscus. 

 Ryder, too, adopts a questionable view of the germinal cavity, when he says that it is 

 " simply a space filled with fluid, which facilitates the gliding of the blastoderm over the 

 yelk during growth," and constituting the fissure between the outer (embryonic) layer 

 and the inner envelope of the yolk, and further as the representative of a " primal 

 nutritive space," a lymph-cavity. He also considers that the body-cavity is continuous 

 with the segmentation-cavity, and maintains that it does not disappear in Gaclus 

 morrhua, Cyhium, Coregonus, and Alosa. 



While there are many points, therefore, which support the view that the segmentation- 

 and germinal cavities are not one, but may indeed co-exist, or may appear successively 

 in the same ovum, there is a possibility that the difference between the deep-seated cavity, 

 seen, for example, in Elasmobranchs, and the sub-blastodermic chamber in Teleosts, may, with 

 extension of our knowledge of the early blastoderm in the latter, disappear, and this would 

 be so if it could be shown that the germinal cavity arises, not by the lifting up of the 

 disc, but by intracellular dehiscence, and the disappearance of the lower (separate) 

 stratum, i.e., the blastomeric floor.* At present the germinal cavity must be distinguished 

 as such, the characteristic features being its situation superficial to the yolk, the absence 

 of blastoderm-cells separating it from the granular yolk-cortex, and its persistence even 

 into the later embryonic period. Other minor features justify us in emphasising the 

 distinction of this cavity from the blastoccel or segmentation-cavity proper. 



VI. Periblast or Nuclear Zone. 



From the way in which the protoplasm of the ovum collects at the animal pole, it 

 is readily seen that the continuity of the disc and the cortical protoplasm beyond does 

 not cease for some time, and that even when the blastoderm by cleavage has become 

 defined in the form of a cellular prominence, its connection with the unsegmented 

 protoplasm external to it is most intimate. The process of superficial transference still 

 proceeds after cleavage has commenced, t 



* The fact that during a considerable interval the segmentation-cavity in Elasmobranchs is greatly deficient in its 

 cellular floor, and the yolk limits it below (No. 11, p. 518), is interesting, though Balfour doubts if ever the yolk alone 

 forms the floor (p. 519). Gotte's observations would demonstrate the existence of such a floor of cells in the Tele- 

 osteans, though it is always incomplete. 



t Granular yolk is also transferred in the Elasmobranch, both Oellacher and Balfour agreeing that yolk is 

 assimilated by the germinal area during segmentation. The cessation of the transference and of the yolk cell-gemma- 

 tion accounts in a great measure, according to Balfour, for the comparative distinctness of the disc and the yolk at the 

 end of segmentation. 



