278 



NERVOUS SYSTEM OF VERTEBRATES. 



position. It will be seen that while in the adult the hypothalamus 

 lies in an extreme ventral position, somewhat caudal to the nucleus 

 habenulae, its embryonic and presumably early phylogenetic 

 position was one neuromere cephalad from the nucleus habenulae 

 and not so far ventrad as in the adult. On the other hand, while 

 the nucleus habenulae in the adult Hes at the dorsal border of the 

 brain wall it was overtopped in primitive vertebrates by the 

 somatic sensory center. When it is considered that the large center 

 for the tractus strio-thalamicus and the central gray have separated 

 the hypothalamus and nucleus habenulae, that the hypothalamus 



Plex. chor. 



Tr. habenulo-peduncularis 



Fig. 140. — Transverse section of the nucleus habenulae of the sturgeon. 



has protruded ventrally as the result of its expansion, and that 

 the nuclei habenulae have been drawn dorsally by the habenular 

 commissure connecting them, it becomes altogether probable 

 that these two tertiary olfactory centers have been developed 

 from the same column of indifferent material, namely, the sub- 

 stantia reticularis grisea of the second and third neuromeres. They 

 are morphologically not dorsal and ventral structures, but represent 

 the substantia reticularis of successive segments. Both receive 

 identical tracts, including fornix fibers, which have been bifur- 

 cated by the mechanical shifting apart of their end-nuclei. 



