THE DIENCEPHALIC FLOOR Pho 
In surface relief, however, it forms a prominent element in their 
illustration of the diencephalic floor in this form. Sterzi (21) 
in his illustrations of Acanthias vulgaris (80 em. long), in Mus- 
telus laevis (30 cm. long) and in Raja clavata (60 em. long) 
shows a similar structure dorsal to the saccus vasculosus and 
ventral to the posterior lobe although in none of these cases 
has he included this element in his description. Burckhardt 
(24) shows a similar condition in Protopterus annectans. In 
the ontogenesis of the dog-fish, of the chick and of the cat the 
dorsal evagination of the infundibular region is a constant ele- 
ment and may be traced through successive stages until the 
definitive post-infundibular eminence has made its appearance. 
Thus the embryological history of the infundibular region seems 
to make clear the fact that the inferior lobes may be homologized 
with the post-chiasmatic eminence. The infundibular process, 
including as it does the saccus vasculosus of the ichthyopsid, 
is the homologue of the infundibular process in the sauropsid 
-and mammal, although in these latter forms the saccus formation 
is retrogressive or absent. In this respect the writer agrees with 
Johnston (11) and concurring with him can find no evidence to 
support Edinger’s (18) idea as embodied in his schematic figure 
of a sagittal section of the vertebrate brain which shows an 
infundibular process in contact with the pituitary gland while 
dorsal to it is an entirely separate evagination of the brain floor 
which he calls the saccus vasculosus. It seems equally clear 
that the homology of the post-infundibular eminence may be 
established throughout the phylum. That it is an element 
separate and distinct from the saccus vasculosus is evident 
from the ontogeny of the selachian in which both a saccus vas- 
culosus and a post-infundibular eminence are present. There 
can be no grounds, therefore, for the homology suggested by 
Retzius (9) between the ‘saccular eminence’ of mammals and 
the saccus vasculosus of fishes. 
Although the fact has not been fully established, the evidence 
furnished by the ontogenesis of the dog-fish, of the chick and 
of the cat strongly suggests that the derivatives of the three 
segments of the ectoptic zone are codrdinate. In this light the 
