66 The C/ianyes in the Blastopore of the Newt. [May 29, 



traced into connexion with the middle part of the gut. (In longi- 

 tudinal sections it appears as a mere line, but its continuity with the 

 rest of the gut can then be clearly followed.) 



For some distance behind the blastopore all the layers are fused in 

 the middle ventral line. We find, in fact, a primitive streak, exactly 

 comparable with that of the amniotic vertebrata. Passing further 

 backwards,, a large solid roundish mass is gradually marked off in the 

 middle line from the lateral plates of mesoblast and the hypoblast 

 yolk-cells, This mass gradually acquires a lumen, and is finally con- 

 tinued round into the medullary canal on the dorsal surface. Thus 

 there is no neurenteric canal at this time, and I have equally failed to 

 find it at any other stage. 



The tail next buds out from the region behind the blastopore. It 

 is a small roundish knob, its apex pointing downwards and forwards 

 on account of the ventral curvature of the whole body. Scott and 

 Osborn describe it as " an unsegmented mass of mesoblast." I should 

 prefer to call it an outgrowth of the front end of the primitive streak, 

 in which the same features that I have described in the previous 

 stage are now seen still more clearly. -As it passes back from the 

 dorsal surface round the hind end of the body, the medullary canal 

 loses its lumen and becomes a solid cord of epiblast, distinct from 

 the notochord and lateral plates of mesoblast. A few sections 

 further on, it becomes indistinguishably fused with both these struc- 

 tures, and a mass of cells is thus formed, from which a knob repre- 

 senting the rudiment of the notochord projects into the central yolk- 

 cells. Passing still in the same direction along the ventral surface 

 towards the blastopore, the notochordal rudiment disappears and the 

 fused mass of cells becomes shorter from above downwards, and also 

 compressed from side to side. The cavity of the hind gut is almost 

 obliterated in the hind part of the ventral side of the body, and then 

 widens out again in front. Here it is a slit, having its dorsal wall in 

 close contact with its ventral wall, but comparatively broad from side 

 to side. Further forwards it becomes wider from above downwards 

 and narrower from side to side, till it passes at the root of the tail 

 into the blastopore. The blastopore marks the extreme front end 

 of the primitive streak on the ventral surface. 



The solid condition of the hinder part of the medullary canal 

 for a short region before it becomes indistinguishably fused with 

 the primitive streak is described by Crasser in the bird, and he asserts 

 that the lumen is gradually continued back as the medullary canal is 

 differentiated out of the primitive streak. The description would, 

 I think, apply equally well to the newt. 



I have not yet made out satisfactorily the condition of the primitive 

 streak in the early stages, but I hope to be able to elucidate this later, 

 and to publish figures illustrating my results. 



