THE DIENCEPHALIC FLOOR Dey 
is less convoluted and less vascular, while the ventral one has 
increased in thickness. 
In sauropsids, for birds as well as reptiles, the infundibular 
process differs in certain details from that of the ichthyopsid 
although the general homology of the structure in all these forms 
is discernible. In all three instances the cavity of the third 
ventricle extends into the infundibular process. In the sela- 
chian and amphibian this communication is not defined by 
any marked constriction. The bird (Gallus gallus; fig. 1, C) 
on the other hand, shows a distinct constriction in the region 
where the cavity of the general ventricular chamber passes over 
into the recess of the infundibular process. The process still 
presents its two characteristic surfaces: the pituitary surface 
is in contact with the pituitary gland, while the dorsal or saccular 
surface is much convoluted and non-vascular. This surface, 
unlike that in the frog and dog-fish, is thick. The recess of the 
infundibular process in the bird as in the other forms already 
described presents numerous branching diverticula. 
The mammalian structure is characterized by a marked 
change in that the cavity of the third ventricle does not extend 
“into the infundibular process which latter, in consequence, be- 
comes solid except for a small proximal portion of its stem. 
These conditions are shown in the dog (fig. 1, E). So far as 
I am able to state at present one family alone, the Felidae, 
departs from the mammalian type in this respect. Here the 
third ventricle actually communicates with a large recess in the 
infundibular process by means of a narrow, tubular canal which 
passes from the ventricle through the stem of the process (fig. 1, 
D). The recess of the infundibular process shows no branch- 
ing diverticula; the walls which bound it are thick and non- 
vascular, so that from all appearances it may be inferred that the 
convoluted saccular surface, so conspicuous in the selachian, 
amphibian and sauropsid, has been replaced by a now very 
extensive pituitary surface. This supposition is rendéred more 
probable by the fact that the entire infundibular process in the 
mammal is completely invested by the tissue of the pituitary 
gland. In the anthropoids and man (fig. 1, F) the solidification 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 25, NO. 3 
