DEVELOPMENT OP PETROMYZON FLUVIATILIS. 333 



shallow groove is seen running forward from the blastopore, 

 round about two thirds of the embryo and passing a little in 

 front of the blind end of the mesenteron. The groove is a very 

 temporary structure and is soon replaced by a ridge. This 

 arises by the epiblastic cells lining the groove, which are of a 

 columnar shape, budding off cells from their under surface. 

 The result of this is that a keel of cells is formed which forms 

 the neural ridge externally (fig. 12), and internally presses in 

 between the raesoblastic plates. The keel arises solely by the 

 epiblast cells budding off cells in their under surface only. It 

 is much deeper in the anterior third of its course, which region 

 ultimately forms the brain. 



The keel in the course of two or three days loses its connec- 

 tion with the epidermis ; this occurs at first anteriorly and ex- 

 tends backward, and as Scott has pointed out, it does this of 

 itself and not by an ingrowth of the mesoderm in each side as 

 Calberla described. 



Figs. 13, 15, and 16 show the solid neural cord lying above 

 the notochord, which by this time is separated off from the 

 hypoblast. It is important to notice that the neural canal 

 does not arise until after the connection between the neural 

 cord and epidermis is severed. It is about the origin of this 

 neural canal that my observations and those of Calberla and 

 Scott are at variance. They described the epidermic layer 

 of epiblast passing down into the nervous, in such a way that 

 the canal, when it does appear, is lined by this layer. I have 

 not been able to see any trace of this. The cells forming the 

 nervous system appear to me to be all split off from the under 

 surface of the epidermis in the dorsal middle line, and the 

 continuity of the epidermis in this region never seems to be 

 broken by any such invagination as they suggest. Balfour 

 was also doubtful on this point; but in his and Parker's 

 work on the development of Lepidosteus, they state that 

 there is no evidence of the epidermic layer being concerned in 

 the formation of the canal. 



The canal seems to arise as a split between the cells in the 

 axis of the solid cord, and not by the absorption of the central 



