458 Journal of Comparative Neurology and Psychology. 



II. Discussion 504 



1. The anterior end of the head and brain 504 



2. The homology of the saccus vasculosus 50G 



;>. Segmentation of the neural tube in front of the cerebellum. 507 



4. Boundary between diencephalon and telencephalon 508 



5. The ventricles and the tela 513 



G. Dorsal and ventral zones in the diencephalon and telen- 

 cephalon 510 



7. Pallium of the telencephalon 51j8 



8. Divisions and nomenclature 525 



Summary and conclusions 532 



Proposed revision of the BNA 533 



111 all classes of vertebrates the forebrain vesicle of the early 

 embryo gives rise to two secondary brain segments, the diencephalon 

 and telencephalon. The morphological features of the diencephalon 

 are remarkably constant throughout the vertebrate series, although 

 this segment is the most peculiar and irregular in form. There is 

 always a membraneous roof, tela chorioidea and one or two epiphyses. 

 The dorsal border of the lateral wall presents a paired enlargement 

 known as the ganglion habenulse, to which comes from the telen- 

 cephalon a tract of fibers which often appears as a gross feature — 

 the stria medullaris. The massive lateral walls bound a third ventricle 

 wdiich is narrow from side to side. In these lateral walls develop 

 a considerable number of special centers of which the lateral genic- 

 ulate bodies are recognizable microscopically in fishes. The ventral 

 wall of the diencephalon is depressed and variously shaped according 

 to the degree of development of the olfactory and gustatory organs. 

 From in front the optic tracts enter through the base and run up 

 in the lateral walls. Behind the chiasma is found always in the 

 embryo and usually in the adult (fishes, amphibians, reptiles) a 

 transverse groove or depression known as the recessus postopticus. 

 Behind this is a wider depression which in mammals is provided with 

 thick walls, the tuber clnereum, but in fishes and amphibians is a 

 broad bilobed stnicture with thinner walls, concave within, known 

 as the lohi inferiores. Between these and continuing backward and 

 downward from them is a thin walled sac, the saccus vasculosus. 



