346 JORDAN. [Vol. VII L 



figures i, 2, 3) into a stage of apparent fusion. I put this 

 forward, however, only as a mere suggestion to be tested by 

 future observation of Anuran embryos of this stage. Schultze's 

 figures do not convince me that the "fusion" along the median 

 dorsal line in Rana is in any way the equivalent of the primi- 

 tive streak in DiemyctyliLS. {Cf. e.g. Figs. 58 and 65 with 

 Schultze's Figs. 5, 6, 7.) 



The neural groove, which is always quite shallow along most 

 of its extent, becomes, soon after its appearance, much deeper 

 , in the extreme anterior portion. This deepening is evidently 

 the "pit" mentioned by Miss Johnson ('84, p. 662). There is, 

 as will be seen in Fig. 64, an apparent fusion of layers at 

 the base of this pit. I hesitate, however, to attribute any 

 special significance to this fusion since it is isolated and 

 limited in extent, and since I am convinced that the appear- 

 ance of fusion is due chiefly to the dipping in of the superficial 

 ectoblast. 



The meaning of the neural groove is, I confess, very puz- 

 zling. I am inclined to think the groove may be caused 

 by the mechanical stresses that attend the development of the 

 neural folds, but am poorly satisfied with so nebulous an ex- 

 planation. I am compelled, nevertheless, to accept some 

 such interpretation rather than any that would relate it to the 

 primitive groove, from which I believe it to differ utterly in 

 origin, structure and significance. It will be seen, then, that 

 I regard the primitive streak of Amphibia as formed by fusion 

 of the blastodermic layers, and that this fusion is given a linear 

 direction by the coming together of the lateral blastopore lips. 

 In this respect I agree with Schwarz ('84), Goette,^ and Robin- 

 son and Assheton ('9l). I agree also with these last-named 

 authors that there is no reason for not including the ventral lip 

 of the anus within the primitive streak. The whole germ-ring, 

 the fused rim of the blastopore, is responsible for the forma- 

 tion of the primitive streak, and the latter structure is simply 

 the result of the fusion of the blastopore lips along the axial 



1 1 have not been so fortunate as to see Goette's paper (Abhandlungen zur 

 Entwickelungsgeschichte der Tiere, V. Heft, Leipzig, 1890) and know it only by 

 abstracts. 



