THE DIENCEPHALIC FLOOR 269 
peduncle ultimately becomes expanded and contains a large 
recess accessory to the third ventricle which overlies the optic 
chiasm and proximal portion of the optic nerve. The remainder 
of the optic diverticulum rapidly assumes the characteristic 
form of the eye-cup, while the cavity between its ental and ectal 
layers communicates with the third ventricle by means of the 
optic peduncle, retaining this communication until a late period 
of embryonic life. Ultimately the lumen of the distal portion 
of the peduncle becomes obliterated. In the adults of all the 
forms examined a marked prechiasmatic recess of the ventricle 
is present. It is continued laterad along the dorso-cephalic 
margin of the chiasm and for some distance above the optic 
nerve as the supraoptic recess. The position of these recesses 
is indicated upon the surface by a ridge which traverses the chiasm 
and proximal portion of the nerve, the supraoptic crest. This 
likewise is constant in all the forms examined. Osborn (13), 
Herrick (14) and Kingsbury (15) have shown that the rudi- 
mentary condition of the eye in Necturus is accompanied by a 
similar condition of the optic nerve which retains the primitive 
lumen of the optic vesicle and is hollow for a considerable dis- 
tance peripherad. This fact and the embryological evidence 
make clear the homologies of the supraoptic crest, chiasmatic 
process, prechiasmatic and supraoptic recesses in the adult. 
The infundibular region. The infundibular region presents 
greater difficulties for analysis. ‘These doubtless are due to the 
fact that the ventral segment of the ectoptic zone, like the 
cephalic and dorsal segments, is capable of marked adaptive 
variations. As seen in the elasmobranch, for example, the main 
feature of this region is the so-called hypoariwm described and 
first so named by Sanders (16). The hypoarium consists of 
two large symmetrical eminences situated one on either side of 
the mid-sagittal line immediately caudad to the optic chiasm and 
containing an extension of the third ventricle. In their general 
conformation they are not unlike the optic lobes of the mid- 
brain; they are usually termed the lobi inferiores. In many 
teleosts these lobes are still further subdivided forming on either 
side a large lateral lobe and a small inferior lobe, both of which 
