662 ALICE JOHNSON. 



tive " of Bambeke^ and may obviously be called the primitive 

 groove. Hertwigi says that it is at all stages sharply marked 

 off from the blastopore by an intervening ridge of cells. In 

 my specimens this sometimes occurs_, but it happens at least as 

 frequently that the groove is continuous with the blastopore at 

 its first appearance, and I always find them continuous after 

 the formation of the medullary folds. 



Transverse sections through an embryo with a primitive 

 groove and before the medullary folds have been formed shows 

 that in the region of the groove the three embryonic layers are 

 continuous with one another (vide fig. 1, which represents 

 a section taken through about the middle of the embryo). It 

 happened in this embryo that the primitive groove was con- 

 tinuous with the blastopore. In the anterior part of the 

 embryo the groove flattened out and gradually disappeared. 

 Fig. 2 represents a section through the groove near its anterior 

 end, and shows that here the epiblast is distinct from the other 

 two layers, the mesoblast still retaining its connection with the 

 hypoblast. The mesoblast has generally been described (viz. 

 by Scott and Osborn, Hertwig, and Bambeke^) as being derived 

 exclusively from the hypoblast, except at the blastopore, from 

 the lips of which it grows. It appears to me, on the contrary, 

 that the greater part of it is derived from the primitive streak 

 as in the higher Vertebrates, for it is seen in fig. 2 that the 

 mesoblast cells, where they are represented as derived from 

 the hypoblast, are much fewer in number than appears in 

 fig. 1, where they are shown growing out from the primitive 

 streak. 



The primitive groove in another embryo of a slightly later 

 stage exhibits a deep pit at its anterior end. I am unable at 

 present to state whether any fusion of the layers exists in the 

 region of this pit at this time. 



The next step forwards in development consists in the for- 



1 0. Hertwig, loc. cit. . 



2 Ch. van Bambeke, " Formation des feuillets embryonnaires et de la Noto- 

 corde cliez les Urodeles," ' Bulletins de I'Acad. Boy. de Belgique/ 2me serie, 

 tome 1, 1880. 



