508 'Journal of Comparative Neurology and Psychology. 



thalamic nerve of Miss Piatt which probably forms the ciliary gan- 

 glion, and the part of the neural crest which forms the ophthalmic 

 division of the trigeminus. The terminal part of the neural crest in 

 close relation with the neuropore presumably gives rise to the 

 ganglion of the nervus terminalis in selachians. If this be true, every 

 neuromere of the embryonic brain has connected with it in one class 

 of vertebrates or another some sensory nerve or sense organ (includ- 

 ing the optic vesicle and epiphysis. The five brain segments are 

 equally clearly to be seen in Figs. 34 and 35. 



If. Boundary between Diencephalon and Telenceplialon. — The pos- 



FiG. 42. Pig embryo, 7 mm. Two parasagittal sections to show the seg- 

 ments of the forebraiu and mid-brain. Compare figs. 34, 35 and 18. 



terior boundary of the diencephalon has never been in dispute. It 

 is the constriction between the forebrain and midbrain vesicles and 

 is later marked dorsally by the posterior commissure and ventrally 

 by the tuberculum posterius. When the forebrain vesicle becomes 

 divided into diencephalon and telencephalon the exact location of 

 the boundary between them has not been entirely clear. In all verte- 

 brates in which a definite velum transversum is recognizable this is 

 considered as the mark of the boundary. The existence of a 

 paraphysis and lateral plexus chorioideus in front of the velum and 

 of a dorsal sac and one or two epiphyses behind it is now so thor- 



