DEVELOPMENT OF ELASMOBRANCH FISHES. 95 



the view is adopted that the notochord has in reality a meso- 

 blastic 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 h}^oblast, 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 be- 

 come 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 clear- 

 ness 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 noto- 

 chord afford a strong presumption against its h3"poblastic origin. 



In the second place, the remarkable investigations of 

 Hensen^, 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 proto vertebra the 

 hypoblast becomes considerably thickened beneath the medul- 

 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. 



1 Zeitschrift f. Anat. u. Entwicklungsgeschichte, Vol. i. p. 366. 

 " ^'Sitz. der Gesell. zu Marhurq, Jan. 1876. 



