312 DEVELOPMENT OF ELASMOBRANCH FISHES. 



lary groove ; and, though he has not followed out all the steps of 

 the process by which this thickening is converted into the noto- 

 chord, yet his observations go very far towards proving that it 

 does become the notochord. 



Against the observations of Hensen, there ought, however, to 

 be mentioned those of Lieberkiihn '. He believes that the two 

 lateral masses of mesoblast, described by Hensen (in an earlier 

 paper than the one quoted), are in reality united by a delicate 

 layer of cells, and that the notochord is formed from a thickening 

 of these. 



Lieberkiihn gives no further statements or figures, and it is 

 clear that, even if there is present the delicate layer of meso- 

 blast, which he fancies he has detected, yet this cannot in any 

 way invalidate such a section as that represented on PI. X. fig. 

 40, of Hensen's paper. 



In this figure of Hensen's, the hypoblast cells become dis- 

 tinctly more columnar, and the whole layer much thicker im- 

 mediately below the medullary canal than elsewhere, and this 

 independently of any possible layer of mesoblast. 



It appears to me reasonable to conclude that Lieberkiihn's 

 statements do not seriously weaken the certainty of Hensen's 

 results. 



In addition to the observations of Hensen's on Mammalia, 

 those of Kowalevsky and Kuppfer on Ascidians may fairly be 

 pointed to as favouring the hypoblastic origin of the notochord. 



It is not too much to say that at the present moment the 

 balance of evidence is in favour of regarding the notochord as a 

 hypoblastic organ. 



This conclusion is, no doubt, rather startling, and difficult to 

 understand. The only feature of the notochord in its favour is 

 the fact of its being unsegmented 2 . . 



Should it eventually turn out that the notochord is developed 

 in most vertebrates from the mesoblast, and only exceptionally 

 from the hypoblast, the further question will have to be settled 



1 Sits, der Gesell. zu Marburg, Jan. 1876. 



2 In my earlier paper I suggested that the endostyle of Ascidians afforded an 

 instance of a supporting organ being derived from the hypoblast. This parallel does 

 not hold since the endostyle has been shewn to possess a secretory function. I 

 never intended (as has been imagined by Professor Todaro) to regard the endostyle 

 as the homologue of the notochord. 



