276 FREDERICK TILNEY 
telencephalon derived from the cephalic segment, the thala- 
mencephalon from the dorsal segment and the infundibular 
region from the ventral segment are developmental equivalents. 
It has been shown that from the infundibular region in the 
ichthyopsid the inferior lobes, the infundibular process and the 
post-infundibular eminence are derived. If, as has been assumed 
to be the case in fishes, the inferior lobes are chiefly concerned 
in the gustatory sense, the telencephalon in the olfactory sense 
and the thalamencephalon in the somaesthetic senses, then 
there is further reason to believe that the derivatives of the three 
segments of the ectoptic zone are functionally of the same order. 
The disappearance of the inferior lobes in passing from the 
ichthyopsid to the sauropsid is, as a process, no more difficult 
to comprehend than the similar disappearance of the optic 
lobes of the midbrain in the transition from the bird to the 
mammal. In each instance this process seems to be accomplished 
by the addition of neopallial areas which assume the functions 
of the more primitive brain parts. ~The significance of the per-. 
‘ sistence of the infundibular process and the post-infundibular 
eminence is less clear, although in the former case this doubtless 
is involved in the adaptive variations of the pituitary gland, 
while in the latter the post-infundibular commissure may be a 
determinative factor. 
The tuber cinereum is usually described as an area in the basal 
region of the diencephalon, bounded cephalad by the optic chiasm, 
caudad by the mammillary bodies and laterad by the optic 
tracts and cerebral peduncles. This area in mammals includes 
the post-chiasmatic eminence, the lateral eminences and the post- 
infundibular eminence. From the embryological standpoint, 
the tuber cinereum comprises all of the derivatives of the in- 
fundibular region except the infundibular process and its stem. 
To hold the tuber to this interpretation in fishes and amphibia 
would necessitate the inclusion in it of the inferior lobes, but 
since this has no apparent advantage in the lower forms men- 
tioned it is, perhaps, well to confine the term tuber cinereum to 
mammals, in which instances it is useful in referring to a dis- 
tinctive region of the diencephalic floor. The homology of the 
