342 t. M. BALt^OtJlt,. 



than the other two, we should avoid having to admit its hypo- 

 blastic origin. 



Professor Huxley, to whom I have shown my specimens, 

 strongly advocates this view. 



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 verte- 

 brates. In favour of this view are the following con- 

 siderations : 



(1) 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 mesoblast, which are formed coincidently with the 

 hypoblast by a homogeneous mass of cells becoming differen- 

 tiated 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 hypo- 

 blast and budded off into the mesoblast. We should 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, how- 

 ever, for that reason necessarily 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 arguments against the hypo- 

 blastic origin of the notochord are sufficient to counter- 

 balance the natural interpretations of my sections, cannot, 

 1 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 

 illustrating the possible way in which an organ might come 

 to change the layer to which it primarily belonged. 



If the notochord is originally a mesoblastic structure, it 

 is easy to be seen how, by becoming separated from the hypo- 

 blast a little later than is the case with the Uog-fish, its true 

 mesoblastic origin would become lost ; while if, on the other 



