THE DIENCEPHALIC FLOOR 2a 
growth and thickness in the ventral wall of the infundibular 
process. This, together with the fact that in the Felidae an ex- 
tension of the third ventricle is contained in the infundibular 
process, points strongly to the conclusion that the dorsal wall of 
this process in embryonic stages may be homologized with the 
saccular surface just described in the other forms. That this 
area in mammals becomes invested by the pars infundibularis 
of the pituitary gland, may be interpreted as causing the retro- 
gression of the saccular surface and its replacement by a new 
area in contact with tissue of the pituitary gland, due to the 
greater extension of the infundibular portion of this gland in 
mammals. 
Discussing the significance of the saccus vasculosus and its 
more characteristic formation in the water-living types of rep- 
tiles, Edinger (18) suggests that it may be an apparatus of 
especial importance to aquatic animals. This inference, how- 
ever interesting, is not borne out by the facts observed in some 
of the aquatic mammals, since there is no evidence of anything 
corresponding to the saccus vasculosus or even an abortive 
saccus-formation in Castor canadensis or in Macrorhinus angus- 
tirostris.» The absence, therefore, of any attempt to revert 
to the formation of a saccus vasculosus in mammals, even 
though they be water-living, fails to corroborate Edinger’s 
suggestion that this apparatus is an adaptation peculiar to 
aquatic life. It seems more probable that the disappearance 
of the saccus vasculosus depends upon a profound remodelling ~ 
of the forebrain which occurs in passing from ichthyopsid to 
the sauropsid and mammalian forms. It is not unlikely that 
this highly vascularized structure is closely related to if not 
identical with the chorioid formations and represents in the fish 
a means of supplying an extensive chorioidal plexus to the third 
ventricle, particularly as such plexuses are relatively small in 
other parts of the diencephalon. 
The dorsal evagination of the infundibular region still remains 
to be considered. As already stated, this evagination in the cat 
is of greater size than the ventral one from which latter the in- 
fundibular process is derived. In the bird and the dog-fish 
