MORPHOLOGY OF THE FOREBRAIN 419 



to determine from sections in these specimens, on account of the 

 strong diencephalic flexure. The caudal lip of the velum trans- 

 versum passes over into the diencephalic plexus and, farther 

 laterally and ventrally, into a ridge which forms a massive con- 

 necting bridge between the thalamus and the hemisphere which I 

 shall term the eminentia thalami. This is bounded dorsally and 

 ventrally by sharp ependymal grooves (fig. 5) which converge 

 anteriorly to the interventricular foramen. The dorsal one of the 

 these I shall term the sulcus diencephalicus medius, the ventral 

 one the sulcus diencephalicus ventralis. The sulcus medius ex- 

 tends caudad through the whole length of the diencephalon, turn- 

 ing ventrally behind to join the sulcus limitans in front of the 

 tuberculum posterius. As we shall see beyond (pp. 431 and 469) 

 the sulcus medius is functionally an extension of the sulcus 

 limitans (which ends in the preoptic recess), and the two sulci 

 together in the diencephalon are sometimes termed sulcus Mon- 

 roi. Kupffer ('06, p. 181, fig. 193) designates in Necturus the 

 sulcus ventralis as sulcus Monroi, but this is evidently inap- 

 propriate, for this ventral sulcus separates the hypothalamus from 

 the eminentia thalami (see pp. 431 and 469 ft\). Immediately be- 

 hind the anterior commissural ridge in Amblystoma the ventral 

 sulcus divides. One part follows the caudal border of the ridge 

 into the preoptic recess. This is the ventral part of Kupffer's 

 sulcus interencephalicus anterior (cf. '06, p. 175, fig. 187). The 

 other part continues caudad dorsal to the preoptic nucleus and 

 chiasma ridge to terminate blindly in the caudal part of the hypo- 

 thalamus. It marks the boundary between the preoptic nucleus 

 and hypothalamus and the thalamus in front and at its posterior 

 end separates the hypothalamus proper from the tuberculum 

 pOsterius, which probably belongs morphologically with the pars 

 ventralis of the mesencephalon . 



Still farther dorsal is a short sulcus extending (morphologically) 

 caudad and dorsad from the interventricular foramen and forming 

 the ventral boundary of the habenula. It is the sulcus dience- 

 phalicus dorsalis. 



These relations come out much more clearly in older larvae 

 and adults after the straightening *of the diencephalic flexure. 



