MIDBRAIN AND THALAMUS OF NECTURUS 231 



as a persistent portion of the anterior end of the ernbryonic 

 sulcus Hmitans. 



The hypothalamus of Necturus may be divided on topo- 

 graphic grounds into two general regions, which I shall t^rm the 

 pars dorsalis and the pars ventralis. The pars dor salts hypothal- 

 ami is a massive ridge through which the hypothalamus is 

 connected with the pars ventralis thalami in front and the 

 nucleus of the tuberculum posterius in the floor of the midbrain 

 (figs. 7, 8, 9, 29, 30, 57, 63, 64, p.d.hyth.). It is comparable 

 with the ' Haubenwulst' of Gaupp's description of the hypo- 

 thalamus of the frog ('99, p. 78). The pars ventralis hypothal- 

 ami is a more extensive thickening extending backward from 

 the chiasma ridge in the floor and lateral walls of the infundib- 

 ulum. At its rostral end the region occupied by the fibers of 

 the postoptic commissure in the extensive chiasma ridge is. 

 the urodele equivalent of the pars subchiasmatica of the lobus 

 infundibularis of Gaupp ('99, p. 71). Farther caudad the pars 

 ventralis is enlarged on both sides of the median plane to 

 form the infundibular lobes (figs. 7, 8, 9, 29, 30, 32, 57, 63, 64, 

 p.v. hyth.). 



The hypothalamus is in very intimate relation with the pre- 

 optic nucleus in front and with the pars ventralis thalami above. 

 The rostral part of the pars ventralis thalami is separated from 

 the preoptic nucleus by the very deep sulcus ventralis, but this 

 sulcus is interrupted above the chiasma ridge (fig. 63) by a 

 slender connecting bridge between the pars ventralis thalami 

 and the pars ventralis hypothalami. Immediately caudad of 

 this bridge a derp sulcus separates the dorsal and ventral parts 

 of the hypothalamus. The ventral part of the caudal end of 

 the pars ventralis thalami is broadly continuous with the pars 

 dorsalis hypothalami. The boundary between them is marked, 

 however, by a very narrow and shallow transverse sulcus which 

 was clearly seen in the microscopic preparations and in some, 

 but not all, of the gross preparations upon which figure 63 is 

 based. This may be a remnant of the embryonic sulcus limi- 

 tans. The dorsal part of the caudal end of the pars ventralis 

 thalami is similarly related with the nucleus of the tuberculum 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 2 



