No. 2.] THE DEVELOPMENT OF THE NEWT. 345 



there is no such forward extension of the fused mass of 

 layers. 



The length of the primitive streak in Diemyctyliis never 

 exceeds or indeed equals the diameter of the original blastopore. 

 This, too, is despite the probable additions to the fused area 

 due to cell proliferation. I attribute the differences in the 

 length of the primitive streak of Diemyctylus to the variations 

 in the amount of centripetal convergence of the marginal cells. 

 Early meeting of the lateral lips will produce a long primitive 

 streak, late meeting a short one. 



Surface views, as I have already stated, do not show in the 

 newt any trace of a true primitive streak in front of the blasto- 

 pore. There is, however, a wide, shallow groove on the dorsal 

 aspect of the embryo ("Riickenrinne," " sillon median ") which 

 might perhaps be taken for the primitive groove. There is no 

 fusion of layers in this region (Fig. 54, n.g}j, and since I must 

 persist is regarding the fusion of layers as the criterion par 

 excellence of the true primitive streak and groove, I must con- 

 sider this second groove as at least in some degree an inde- 

 pendent structure. I shall designate it accordingly as the 

 neural groove (Figs. 40-42, n.g^. The neural groove at its 

 first appearance is not connected with the primitive groove 

 which lies posterior to it, but is in the early stages separated 

 from it by a ridge of cells (Figs. 40-42). Later on the 

 neural groove extends backward to the primitive groove so 

 as to appear continuous with it. The neural groove appears 

 in sections as a shallow depression of the ectoblast, the layers 

 beneath not being involved (Figs. 53-54). Since the forma- 

 tion of the notochord takes place at about this period it 

 seems possible that the uplifting of the notochord cells may 

 have something to do with the external appearance of a 

 "streak" which is sometimes described. More often only 

 the "groove" is apparent externally. 



I may perhaps be allowed to suggest that the depression of 

 the ectoblast may in itself account for the "fusion" described 

 by Schultze ('88). It may possibly have come about that the 

 sinking in of the neural groove ("/r." in Schultze's figures 5, 

 6, 7) changes the earlier condition of separate layers (Schultze's 



