FORMATION AND FATE OF THE PRIMITIVE STREAK. 119 



The Chorda Dorsalis. 



We have already stated that in liana temporaria 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 

 hypoblastic, 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 temporaria any appearances which would justify the 

 conclusions which Schultze forms from his observations upon Rana 

 fusca, i.e., "Dass sehon auf dem Stadium der begiunenden Gastrulation 

 in der dorsalen Urdarmwand drei Keimbliitter 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 separation 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 ourselves that the line of separation was normal, for we have 

 invariably found in the space between the mesoblast and hypoblast 

 broken fragments of cell, 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 medullai'y 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 commencement of the separation on the right side (S), 

 nor from the hypoblast, and the chorda is fused both with mesoblast 

 and hypoblast. 



At a later period the mesoblastic plates become entirely separated 

 from the dorsal hypoblast, and afterwards the chorda is also separated ; 

 it then forms a distinct rod, which lies free below the floor of the 

 neural groove and above the dorsal hypoblast except at its posterior 

 extremity, where it terminates in the mass of cells which form the 

 anterior wall of the neurenteric canal. 



