MORPHOLOGY OF THE FOREBRAIN 437 



dorsal segment by the vestige of the septum ependymale, the 

 dorsal segment being separated from the overlying primordium 

 hippocampi by a deep ependymal groove which represents an in- 

 complete fissura limitans hippocampi (see Kingsbury, '95, figs. 32 

 and 31). The nucleus medianus septi extends for a very short 

 distance backward above the interventricular foramen, this part 

 being homologous with the pars fimbrialis septi (Kappers, '08) 

 of the frog. In my specimens of Necturus neither the eminentia 

 thalami nor the diencephalic sulci are so distinct as in Ambly- 

 stoma. The other features of this brain so far as noted agree 

 substantially with those of Amblystoma. 



The preparations in the Harvard Embryological Collection 

 show that in the Necturus larva of 29.6 mm., as the nucleus me- 

 dianus septi invades the septum ependymale, it pushes farther 

 backward in the dorsal border of the septum than in the ventral. 

 This is in contrast with the condition found in Amblystoma, as 

 will be seen by a comparison of fig. 24 with fig. 27. In the latter 

 figure the right side is farther caudad than the left, by reason of 

 slight obliquity of the section. The nucleus medianus fills the 

 whole septum on the left side, but on the right has grown back into 

 its dorsal part only. This is the beginning of the pars fimbrialis 

 septi. Two sections (28/*) farther back the septum is wholly 

 membranous (fig. 28), and the interventricular foramen appears 

 in the next section caudad. 



In Kupffer's account ('06) of the larval Necturus 24 mm. long 

 it is evident from the preceding considerations that he has incor- 

 rectly identified some of the structures in the median wall of the 

 hemisphere. In his fig. 191 he designates rostrally of the inter- 

 ventricular foramen a sulcus intermedins said to be equivalent 

 to the fissure so named by Gaupp in the frog and to separate the 

 eminentia pallialis medialis (my primordium hippocampi) from 

 the eminentia septalis (my pars ventro-medialis) . This is impos- 

 sible, for the part here marked eminentia septalis gives rise to the 

 fibers of the hippocampal commissure and conforms in every other 

 respect to the pars dorso-medialis and not to the pars ventro- 

 medialis; i.e., it is equivalent to the primordium hippocampi of 

 the frog and not to the eminentia septalis, as Kupffer supposed. 



