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



neut nuclei, an appearance Avhich has been noticed in 

 many other forms (PL XX, fig. 15). 



This completes the interesting history of the development 

 of the notochord. To summarise : The invagination hypo- 

 blast cells are first continuous as a single layer, -wholly 

 across the median line ; those farthest from the three central 

 cells are gradually pushed down by the ingrowth of the 

 mesoblast. There is no tendency to split below. They are 

 further reflected around until the lateral cells meet, and the 

 continuity with the hypoblast is broken. It still impinges 

 against the epiblast above, and forms the upper boundary of 

 the alimentary canal below, 



A comparison has already been instituted between the 

 development of the notochord in Triton and its development 

 in the Frog, the Lamprey, and the Elasmobranch Fishes. 

 In important details the processes are very similar. To 

 carry the comparison a step further, in Amphioxus the noto- 

 chord is differentiated from the hypoblast before the meso- 

 blast has become constricted off, and at the time that the 

 medullary plate is closing in above. 



Hensen has demonstrated, beyond doubt, that the noto- 

 chord is of hypoblastic origin in the Guinea-pig; and that it 

 probably arises in the same way in the Rabbit. Quite 

 recently," Mr. Balfour has shown that it has a similar deriva- 

 tion in the Lizard, Lacerta tnuralis. 



In several respects the notochord arises in a simpler 

 manner in Triton than in any of those forms in which the 

 process has been clearly folloAved out. In that: first, the 

 cells do not reduplicate vertically, as in the Elasmobranchs 

 and the Lamprey, previous to the formation of the organ ; 

 second, when the organ is completely formed, it still bounds 

 the alimentary canal below, as in neither of the other forms 

 nor in the Frog ; third, no portion splits off subsequently to 

 form the hypoblast layer bounding the canal above, this 

 layer appears from the sides. 



It is difficult to judge from Kowalevsky's description, 

 Avhether the whole depth of the layer bounding the canal 

 above is absorbed by the notochord, or whether the lower por- 

 tion remains as an upper lining of the canal, and the upper 

 portion alone enters into the notochord. If the latter is the 

 case, the Newt presents the simplest notochordal develop- 

 ment known. 



The evidence from all these forms points so strongly in 

 one direction, as to amount almost to proof, that the study 



* Vide Kowalevsky, loc. cit. 



- Vide F. M. Balfour, this Journal, Vol. XIX, p. 3, New Series. 



