We have, I think, no right to assume the truth of this view without further proof. The general admission of assumptions of this kind is apt to lead to an injurious form of speculation, in which every fact presenting a difficulty in the way of some general theory is explained away by an arbitrary assumption, while all the facts in favour of it are taken for granted. It is however clear that no theory can ever be fairly tested so long as logic of this kind is permitted. If, in the present instance, the view is adopted that the notochord has in reality a mesoblastic origin, it will be possible to apply the same view to every other organ derived from the hypoblast, and to say that it is really mesoblastic, but has become separated at rather a late period from the hypoblast. If, however, we provisionally reject this explanation, and accept the other alternative, that the notochord is derived from the hypoblast, we must be prepared to adopt one of two views with reference to the development of the notochord in other vertebrates. We must either suppose that the current statements as to the development of the notochord in other vertebrates are inaccurate, or that the notochord has only become secondarily mesoblastic. The second of these alternatives is open to the same objections as the view that the notochord has only apparently a hypoblastic source in Elasmobranchii, and, provisionally at least, the first of them ought to be accepted. The reasons for accepting this alternative fall under two heads. In the first place, the existing accounts and figures of the development of the notochord exhibit in almost all cases a deficiency of clearness and precision. The exact stage necessary to complete the series never appears. It cannot, therefore, at present be said that the existing observations on the development of the notochord afford a strong presumption against its hypoblastic origin. In the second place, the remarkable investigations of Hensen[189], on the development of the notochord in Mammalia, render it very probable that, in this group, the notochord is developed from the hypoblast. Footnote 189: _Zeitschrift f. Anat. u. Entwicklungsgeschichte_, Vol. I. p. 366. Hensen finds that in Mammalia, as in Elasmobranchii, the mesoblast forms two independent lateral masses, one on each side of the medullary canal. After the commencing formation of the protovertebræ the hypoblast becomes considerably thickened beneath the medullary groove; and, though he has not followed out all the steps of the process by which this thickening is converted into the notochord, yet his observations go very far towards proving that it does become the notochord.