60 W. B. SCOTT AND HENRY F. OSBORN. 



4. The epihlast is at first composed of a single layer of 

 columnar cells, which early separate into two rows, and of the 

 two layers thus formed the inner becomes the active one, 

 entering exclusively into the formation of the sense organs. 

 In the spinal cord and brain the division into two layers does 

 not take place till very much later. 



5. The hypoblast is of two kinds, the invaginated and that 

 which arises from the metamorphosed yolk-cells. 



6. The notochord is of hypoblastic origin, and takes up the 

 entire dorsal wall of the alimentary tract (except in the head) 

 in its formation, fresh hypoblast growing from the sides 

 below it. It becomes well formed and cylindrical in shape 

 before any cell division takes place in it, 



7. The hody-cavity extends into the head, appearing in 

 this region first. The head mesoblast becomes split into 

 somites, which have the same relations and number (except 

 so far as modified by the reduction of the visceral clefts) as 

 in the Elasmobranchs, but do not seem to communicate 

 below. 



8. The thyroid body is formed by an outgrowth from the 

 alimentary canal, the walls of which become continuous with 

 the mucous layer of the epiblast ; the continuity of the horny 

 layer is not interrupted. 



Conclusion, 



If the statements in this paper prove to be well founded, 

 they will give us some data for judging of the relationships 

 of the two groups of Amphibia to each other, and to some 

 lower types. The marked divergences from the Batrachian 

 type which the Newt shows us point to the conclusion that 

 the Urodeles and Batrachians have been separated for a very 

 long period. And it is interesting to observe that, in those 

 cases where the divergence is other than a mere matter of 

 detail, it leads towards the Lamprey, and through that to 

 Amphioxus. The opinion seems to be gaining ground that 

 some such form as the Lamprey is the point toward which the 

 Amphibia, the Elasmobranch, Ganoid, and Dipnoic fishes 

 converge, and the more these types are investigated the 

 better established appears this view. As yet, however, we 

 are not in a position to pronounce upon it with even an 

 approximation to certainty. The observations brought 

 forward in this paper tend strongly, we think, in this 

 direction, and we hope that future investigations upon the 



