DEVELOPMENT OF ELASMOBRANCH FISHES. 89 



yolk, as has been already mentioned, by a simple aggregation 

 of protoplasm around pre-existing nuclei. 



The cells being described are in most cases formed close to 

 the pre-existing hypoblast cells, but often require to undergo a 

 coDsiderable change of position before attaining their final 

 situation in the wall of the alimentary canal. 



I have already alluded to this feature in the formation of 

 the ventral wall of the alimentary cavity. Its interest, as bear- 

 ing on the homology of the yolk, is considerable, owing to the 

 fact that the so-called yolk -cells of Amphibians play a similar 

 part in supplying the ventral epithelium of the alimentary 

 cavity, as do the cells derived from the yolk in Elasmobranchs. 



The fact of this feature being common to the yolk-cells of 

 Amphibians and the yolk of Elasmobranchs, supplies a strong 

 argument in favour of the homology of the yolk-cells in the one 

 case with the yolk in the other \ 



1 Nearly simultaneously with Chapter III. of the present monograph on the 

 Development of Elasmobranchs^ which dealt in a fairly complete manner with 

 the genesis of cells outside the bla?toderm, theie appeared two important papers 

 dealing with the same subject for Teleostei. One of these, by Professor Bambeke, 

 Embryologie des Poissons Osseux, Mem. Cour. Acad. Belgique, 1875, which 

 appeared some little time before my paper, and a second by Dr Klein, Quart. Jour, 

 of Micr. Sci. April, 1876. In both of these papers a development of nuclei and 

 of cells is described as occm-ring outside the blastoderm in a manner which 

 accords fairly well with my own observations. 



The conclusions of both these investigators differ however from my own. 

 They regard the finely granular matter, in which the nuclei appear, as pertain- 

 ing to the blastoderm, and morphologically quite distinct from the yolk. From 

 their observations we can clearly recognize that the material in which the nuclei 

 appear is far more sharply separated off from the yolk in Osseous Fish than in 

 Elasmobranchs, and this sharp separation forms the main argument for the 

 view of these authors. Dr Klein admits, however, that this granular matter 

 (which he caUs parablast) graduates in the typical food-yolk, though he explains 

 this by supposing that the parablast takes up part of the yolk for the purpose of 

 growth. 



It is clear that the argument from a sharp separation of yolk and parablast 

 cannot have much importance, when it is admitted (1) that in Osseous Fish there 

 is a gradation between the two substances, while (2) in Elasmobranchs the one 

 merges slowly and insensibly into the other. 



The only other argument used by these authors is stated by Dr Klein in the 

 following way. " The fact that the parablast has, at the outset, been forming 

 one unit ^ith what represents the archiblast, and, ^chile increasing has spread 

 i.e. grown over the yolk which underlies the segmentation-cavity, is, I think, 

 the most absolute proof that the yolk is as much different from the parablast as 

 it is from the archiblast." This argument to me merely demonstrates that cer- 

 tain of the nutritive elements of the yolk become in the course of development 

 converted into protoplasm, a phenomenon which must necessarily be supposed 

 to take place on my own as well as on Dr Klein's view of the nature of the yolk. 

 My own views on the subject have already been fully stated. I regard the so- 

 called yolk as composed of a larger or smaller amount of food-material imbedded 



