Yolk in the Hggs of Osseous Fishes. 7 



are broken down and form a syncytium, but that tlie germinal 

 matter is so concentrated at one pole as to have little more 

 connexion with the yolk than that of juxtaposition. The 

 yolk seems to have no essential role in segmentation, but is 

 an appendage to the early germ as to the later embryo. The 

 nature and function of the periblast and cortical protoplasm 

 need not be dwelt upon ; they are continuous with and form 

 part and parcel of the germ. The origin and fate of the 

 nuclei which appear in them is by no means decided. As 

 Klein declared, they are not identical with the yolk nuclei of 

 the Elasmobranch e,^g *, and they probably originate, as 

 Agassiz and Whitman hold, and as Wenckebach's recent 

 researches tend to show f, in the segmented blastoderm itself. 

 We know how greatly the food-yolk, when it crowds seg- 

 menting cells, alters their character and disposition ; and the 

 possibility seems naturally to follow that Avhen, as in the 

 Teleostean ^^gs,, the yolk becomes almost wholly separated 

 from the germ, a less distorted and more primitive condition 

 may be resumed. We can thus understand how, notwitii- 

 standing the great bulk of the yolk, the blastopore in Osseous 

 Fishes is symmetrical, and coincides with the entire inflected 

 margin of the germ, while the germ itself forms, not a blasto- 

 sphere with a transient segmentation-cavity, as well as a 

 permanent enteric invagination, but a concave two-layered 

 gastrula, enclosing or rather arching over a primitive gastric 

 chamber (PI. II. fig. 10, g.c). In this enteric chamber, 

 roofed over by invaginated hypoblast and with a floor of peri- 

 blast (PI. II. fig. 10,perib.)^ the globe of passive yolk-matter 

 (PI. 11. fig. 10, y) is seated, and projects from the blastopore 

 until the free margin of the latter has so far progressed over 

 its surface as to entirely envelop it. It persists in the peri- 

 visceral cavity as a ventral protuberance for some time after 

 the embryo has emerged (Pi. II. fig. 6, y) until it is com- 

 pletely disintegrated and absorbed. 



EXPLANATION OF PLATE H. 



Fig. 1. Ovum of Gadus (sglefinus, fifth hour; four blastomeres nearly 



completed, bl., blastomeres ; c.p., cortical protoplasm passiiig 



to the animal pole ; y, yolk. 

 Fig. 2. Ovum of Petromyzonjluviatilis, about same stage as fig. 1 (after 



Shipley), showing the yolk included in the segmentation pi"o- 



cess, bl., blastomeres ; y, yolk. 



* Quart. Journ. Microsc. Sci. vol. xvi. 1870, p. 128. 

 t Archiv f. mikr. Anat. Bd. xxviii. 1886. 



