94 THE NOTOCHORD. 



A slieath is formed around the notocbord, very soon after its 

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

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

 acid and hsematoxylin. I conclude from its subsequent his- 

 tory, that it is to be regarded as a product of the cells of the 

 notocbord, 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 notocbord 

 become very much flattened vertically, and cause the well- 

 known stratified appearance which characterises the notocbord 

 in lonoitudinal sections. In transverse sections the outlines of 



o 



the cells of the notocbord appear rounded. 



Throughout this period the notocbord cells are filled with 

 yolk spherules, and near its close small vacuoles make their 

 appearance in them. 



An account of the development of the notocbord, substan- 

 tially similar to that I have just given, appeared in my prelimi- 

 nary paper^ 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 notocbord from the hypoblast. 



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

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

 of the notocbord from the hypoblast is due to a secondary 

 process. 



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

 tain that the mesoblast becomes separated from the hypoblast 

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

 the latter become 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 

 which every fact presenting a difficulty in the way of some 

 general theory is explained away by an arbitrary assumption, 

 while all the facts in favour of it are taken for granted. It is 

 however clear that no theory can ever be fairly tested so long 

 as logic of this kind is permitted. If, in the present instance, 



1 Loc. cit. 



