82 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



The other possibility is that the notochord is primitively a 

 true hypoblastic structure which has only by adaptation become 

 an apparently mesoblastic one in the higher vertebrates. In 

 favour of this view are the following considerations : 



(i) That this is the undoubtedly natural interpretation of 

 the sections. (2) That the notochord becomes separated from 

 the hypoblast after the latter has acquired its typical structure, 

 and differs in that respect from the two lateral sheets of meso- 

 blast, which are formed coincidently with the hypoblast by a 

 homogeneous mass of cells becoming differentiated into two 

 distinct layers. (3) That the first mode of looking at the matter 

 really proves too much, since it is clear that by the same method 

 of reasoning we could prove the mesoblastic origin of any organ 

 derived from the hypoblast and budded off into the mesoblast. 

 We would merely have to assert that it was really a mass of 

 mesoblast budded off from the hypoblast rather later than the 

 remainder of the mesoblast. Still, it must be admitted that the 

 first view I have suggested is a possible, not to say a probable 

 one, though the mode of arguing by which it can be upheld 

 may be rather dangerous if generally applied. We ought not, 

 however, for that reason necessarily to reject it in the present 

 case. As Mr Ray Lankester pointed out to me, if we accept 

 the hypoblastic origin of the notochord, we should find a partial 

 parallel to it in the endostyle of Tunicates, and it is perhaps 

 interesting to note in reference to it that the notochord is the 

 only unsegmented portion of the axial skeleton. 



Whether the strong a priori difficulties of the hypoblastic 

 origin of the notochord are sufficient to counterbalance the 

 natural interpretation of my sections, cannot, I think, be decided 

 from the single case of the Dog-fish. It is to be hoped that 

 more complete investigations of the Lamprey, &c., may throw 

 further light upon the question. 



Whichever view of the primitive origin of the notochord 

 is the true one, its apparent origin is very instructive as illus- 

 trating the possible way in which an organ might come to 

 change the layer to which it primarily belonged. 



If the notochord is a true mesoblastic structure, it is easy 

 to be seen how, by becoming separated from the hypoblast a 

 little later than is the case with the Dog-fish, its mesoblastic 



