[115] EMBRYOGRAPHY OF OSSEOUS FISHES. 569 



iug the early stages of development I believe, iu some cases, at any 

 rate, to have been purely the products of reagents. Tbe separation of 

 the cells during cleavage is a very probable occurrence, and originates 

 by the cells pushing and displacing each other somewhat during this 

 process, as suggested by Whitman. The evidence which 1 have been 

 able to gather, both from the living eggs and sections, leads me to tlie 

 conclusion that the Keimhokle of Strieker and CEllacher is the true blas- 

 tocoel of the Teleostean ovum, as Ziegler has more recently urged. The 

 yelk hypoblast is its floor, and the at first two-layered epiblast its roof. 

 Its development is constantly the same in all of the forms studied by 

 me, and I have not yet found any evideuce of the existence of species 

 without the epiblastic or outer covering of the yelk-sack, as asserted by 

 Von Baer. 



I see no valid reason for not regarding the yelk as an active part of 

 the ovuui, through the intermediation of the yelk hypoblast, and it 

 seems evident that the segmentation cavity is simply a space filled 

 with fluid which facilitates the gliding of the blastoderm over the yelk 

 during growth, and that it is placed between the blastoderm and the 

 yelk, with its free nuclei peripherally displaced to a remarkable degree. 

 In other words, I would regard the yelk as an integral part of the egg, 

 taking a share in segmentation only at a very late period. In conse- 

 quence of the almost entirely passive condition of the yelk during the 

 earlier stages, the blastoderm is obliged to spread to an extreme degree, 

 and iu parts becomes remarkably attenuated. On this account I would 

 still hold to the vicAv first expressed in my paper on the development 

 of Tylosurus, that the germ-disk alone is practically the homologue of 

 the whole Batrachian or Marsiiiobranch ovum, since we actually do not 

 find any intimate connection of the yelk with the embryo, excei)t by 

 way of the vascular system, which develops late in most forms. In 

 Alosa the yelk might be removed at any stage without taking away any 

 essential part of the embryo except the floor of the segmentation cavity. 

 The mode of development of the gastrnla and coeloma, is, we find, greatly 

 mo<lified by the i)resence of the yelk, but it is not an active factor in 

 the development of either by means of any process of segmentation. 



The free nuclei of the yelk hypoblast apparently proliferate as the 

 blastoderm spreads. Thej' are, at any rate, at first confined to the 

 germinal pole of the ovum, and are only found at the opposite pole 

 after the yelk-globe has been included by the blastoderm. The infer- 

 ence, therefore, is that they spread and multiply with the lateral growth 

 of the blastoderm. It is these nuclei possibly which are the centers 

 of certain free cells around the margin of the germinal disk when the 

 latter has attained the morula stage, as in Cybiian and Tylosurus, as 

 shown in Fig. 3, PI. XIX, of my essay on the latter form. If such is 

 the case, it is possible that the germinal wall [Keirtncall) at the edge of 

 the blastoderm of the chick is homologous with the yelk hypoblast of 

 t)ie fish ovum. In fact, it is highly probable that there is a yelk liyi)o- 



