FORMATION AND FATE OF THE PRIMITIVE STREAK. 493 



The Chorda Dorsalis. 



We have already stated that in Rana temper aria the 

 chorda is formed from the yolk-cells which lie beneath the 

 neural groove. It is unimportant whether we consider these 

 cells to be mesoblastic or hypoblasticj but we wish to emphasise 

 the fact that the chorda in Rana temporaria is not formed 

 from a layer of mesoblast which has previously separated from 

 the hypoblast. We have been unable to find in Rana tem- 

 poraria any appearances which would justify the conclusions 

 which Schultze forms from his observations upon Rana fusca, 

 i. e. '^ Dass schon auf dem Stadium der beginnenden Gastrula- 

 tion in der dorsalen Urdarmwand drei Keimblatter existiren" 

 (45). 



At a later stage, however, just about the time when the 

 medullary groove is only appearing in the posterior part of the 

 embryo, we have noted, under the low power, an apparent sepa- 

 ration of the mesoblast as a distinct layer across the mid- 

 dorsal line ; but this appearance we have only seen in ova which 

 have been treated with staining reagents, and on examination 

 with higher powers we have never been able to convince our- 

 selves that the line of separation was normal, for we have 

 invariably found in the space between the mesoblast and hypo- 

 blast broken fragments of cells, and we are therefore inclined 

 to the belief that the separation of the layers was in these cases 

 artificial, and that it had been produced by the reagents used 

 in staining. 



In unstained specimens the appearances seen in the mid- 

 dorsal line at the time of the appearance of the medullary 

 furrow are represented in fig. 12. The epiblast and mesoblast 

 are separated by a distinct space {E). Laterally the hypoblast 

 and mesoblast are also distinct from each other {H), but along 

 the dorsal wall of the archenteron the mesoblast is neither 

 separated from the chorda, though there are traces of the com- 

 mencement of the separation on the right side {S), nor from 

 the hypoblast, and the chorda is fused both with mesoblast 

 and hypoblast. 



