96 THE NOTOCHORD. 



Against the observations of Hensen, there ought, however, 

 to be mentioned those of Lieberklihn^ 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. 



Lieberklihn 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 

 w^ay 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 Lieberkuhn'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\ 



Should it eventually turn out that the notochord is de- 

 veloped in most vertebrates from the mesoblast, and only ex- 

 ceptionally from the hypoblast, the further question will have 

 to be settled as to whether it is primitively a hypoblastic or a 

 mesoblastic organ ; but, from whatever layer it has its source, an 

 excellent example will be afforded of an organ changing from 

 the layer in which it was originally developed into another 

 distinct layer. 



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

 instance of a supporting organ heiiig derived from the hypoWast. This parallel 

 does not. hold since the endor;tyle has been shewn to possess a secretory func- 

 tion. I'ncver intended (as has been imagined by Profes;;or Todaro) to regard 

 the endost^'le as the homologue of tlie notochord. 



