STAGES B TO G. THE NOTOCHORD. 31! 



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 state- 

 ments 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 ob- 

 jections as the view that the notochord has only apparently a 

 hypoblastic source in Elasmobranchs, and, provisionally at least, 

 the first of them ought to be accepted. The reasons for ac- 

 cepting 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 1 , 

 on the development of the notochord in Mammalia, render it 

 very probable that, in this group, the notochord is developed 

 from the hypoblast. 



Hensen finds that in Mammalia, as in Elasmobranchs, the 

 mesoblast forms two independent lateral masses, one on each 

 side of the medullary canal. 



After the commencing formation of the protovertebrse the 

 hypoblast becomes considerably thickened beneath the medul- 

 1 Zeitschriftf. Anat. u. Entwicklungsgtschiekte, Vol. i. p. 366. 



