310 DEVELOPMENT OF ELASMOBRANCH FISHES. 



notochord commences to be separated from the hypoblast an- 

 teriorly, and that the separation gradually extends backwards. 



The posterior extremity of the notochord remains for a long 

 time attached to the hypoblast ; and it is not till the end of the 

 period treated of in this chapter that it becomes completely free. 



A sheath is formed around the notochord, very soon after its 

 formation, at a stage intermediate between stages C and D. 

 This sheath is very delicate, though it stains with both osmic 

 acid and haematoxylin. I conclude from its subsequent history, 

 that it is to be regarded as a product of the cells of the noto- 

 chord, but at the same time it should be stated that it precisely 

 resembles membrane-like structures, which I have already 

 described as being probably artificial. 



Towards the end of this period the cells of the notochord 

 become very much flattened vertically, and cause the well-known 

 stratified appearance which characterises the notochord in longi- 

 tudinal sections. In transverse sections the outlines of the cells 

 of the notochord appear rounded. 



Throughout this period the notochord cells are filled with 

 yolk-spherules, and near its close small vacuoles make their 

 appearance in them. 



An account of the development of the notochord, substantially 

 similar to that I have just given, appeared in my preliminary 

 paper 1 on the development of the Elasmobranch fishes. 



To the remarks which were there made, I have little to add. 

 There are two possible views, which can be held with reference 

 to the development of the notochord from the hypoblast. 



We may suppose that this is the primitive mode of develop- 

 ment of the notochord, or we may suppose that the separation 

 of the notochord from the hypoblast is due to a secondary 

 process. 



If the latter view is accepted, it will be necessary to maintain 

 that the mesoblast becomes separated from the hypoblast as 

 three separate masses, two lateral, and one median, and that 

 the latter becomes separated much later than the two former. 



We have, I think, no right to assume the truth of this view 

 without further proof. The general admission of assumptions 

 of this kind is apt to lead to an injurious form of speculation, in 



1 Loc. cit. 



