STAGES B TO G. ALIMENTARY CANAL. 305 



considerable 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 bearing 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 . 



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 blastoderm, there appeared two important papers dealing 

 wiih the same subject for Teleostei. One of these, by Professor Bambeke, " Em- 

 bryologie des Poissons Osseux," Mem. Cour. Acad. Bclgiqitc, 1875, which appeared 

 some little time before my paper, and a second by Dr Klein, Quart, your, of Micr. 

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

 described as occurring 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 pertaining to the 

 blastoderm, and morphologically quite distinct from the yolk. From their observa- 

 tions we can clearly recognise 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 calls parablast) graduates into 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 (i) 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 with what represents the archiblast, and, while 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 certain of the nutritive elements of 

 the yolk become in the course of development converted into protoplasm, a pheno- 

 menon 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 in protoplasm, and the meroblastic ovum as a body 



