Against the observations of Hensen, there ought, however, to be mentioned those of Lieberkühn[190]. 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. Footnote 190: _Sitz. der Gesell. zu Marburg_, Jan. 1876. Lieberkühn gives no further statements or figures, and it is clear that, even if there is present the delicate layer of mesoblast, which he fancies he has detected, yet this cannot in any way invalidate such a section as that represented on Pl. X. fig. 40, of Hensen's paper. In this figure of Hensen's, the hypoblast cells become distinctly more columnar, and the whole layer much thicker immediately below the medullary canal than elsewhere, and this independently of any possible layer of mesoblast. It appears to me reasonable to conclude that Lieberkühn'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[191]. Footnote 191: 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. 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 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. EXPLANATION OF PLATE 10. COMPLETE LIST OF REFERENCE LETTERS. _al._ Alimentary canal. _ch._ Chorda dorsalis or notochord. _ch´._ Ridge of hypoblast, which will become separated off as the notochord. _ep._ Epiblast. _hy._ Hypoblast. _lp._ Coalesced lateral and vertebral plate of mesoblast. _mg._ Medullary groove. _n._ Nucleus of yolk. _na._ Cells formed around the nuclei of the yolk to enter into the ventral wall of the alimentary canal. _nc._ Neural or medullary canal. _pv._ Protovertebra. _so._ Somatopleure. _sp._ Splanchnopleure. _ts._ Mesoblast of tail-swelling. _yk._ Yolk-spherules.