MORPHOLOGY OF THE FOREBRAIN # 429 



and apparently reach the pars ventralis thalami above and behind 

 the optic chiasma. A part of this system decussates in the com- 

 missura pallii anterior. I have not been able to trace any of its 

 fibers into the hypothalamus. This tract was recognized in the 

 frog by P. Ram6n y Cajal and named fornix longus. If any 

 fibers of this tract reach the hypothalamus, these would be able 

 to serve as an aberrant columna fornicis, connecting with the 

 caudal part of the primordium hippocampi, just as the typical 

 columna fornicis connects with its rostral end (see fig. 22); but 

 neither my preparations nor P. Ram6n's figure ('96, p. 249) 

 give any evidence of hypothalamic connections. 



12. Tractus habenulo-striaticus. Scattered medullated fibers 

 leave the stria medullaris to spread through the grey matter of the 

 caudal end of the pars ventro-lateralis hemisphaerii laterally of 

 the anterior commissure This is the region designated corpus 

 striatum by some recent authors Though not fully homologous 

 with the mammalian striatum, it is one of the sources of that struc- 

 ture; accordingly I term the fibers tractus habenulo-striaticus. 



13. Tractus habenulo-thalamicus. Similarly medullated and 

 unmedu Hated fibers in larger number enter the rostral end of the 

 pars ventralis thalami. 



14. Tractus thalamo-habenularis. These fibers pass in diffuse 

 formation between the pars dorsalis thalami and the habenula 

 and the most rostral members of the group are for a short dis- 

 tance joined to the stria medullaris. 



In the 17 mm. specimen we found the posterior pole of the hem- 

 isphere solid and extending but a short distance behind the level 

 of the anterior commissure (fig. 5). At 32 mm. the same is true 

 and the diencephalic flexure still obscures the relations between 

 diencephalon and telencephalon somewhat (figs. 25 and 26). 

 In the adult this flexure has disappeared and the posterior pole 

 has grown far backward laterally of the thalamus (figs. 16 to 20). 

 The primordium hippocampi forms the median wall of the pos- 

 terior pole and the pars dorso-lateralis the lateral wall, but its 

 caudal end is a relatively undifferentiated tissue which is in con- 

 tact with the caudal part of the striatum complex below and which 

 has the following characteristic fiber connections: tractus olfac- 



THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO. 5. 



