274 FREDERICK TILNEY 
the reverse is true. The evagination is present in the early 
stages in the selachian and may be traced through to adult life, 
when it occupies a position immediately ventral of the posterior 
lobe and dorsal to the saccus vasculosus. Similarly in the 
bird it lies dorsal to the infundibular process from its inception 
and maintains this position in the adult. Its development in 
the cat is equally clear; here it forms a protuberance of the 
diencephalic floor caudal to the infundibular stem and in front 
of the mammillary bodies; this has been called by the writer the 
post-infundibular eminence. Much discussion has arisen con- 
cerning this element of the interbrain. Retzius (9), who first 
described it in mammals, considered that it was the homologue _ 
of the saccus vasculosus in fishes. Staderini (20), however, 
states that topographically, as well as from its developmental 
relations, the saccus vasculosus can bear no relation to the 
eminentia saccularis. It is also his opinion that nothing in the 
intimate constitution of the saccular eminence favors the con- 
ception of Retzius. In fishes the saccus vasculosus is a thin- 
walled structure connected with a great number of blood vessels. 
In man and other mammals the structure referred to by Retzius 
is a relatively thick-walled evagination with scanty vasculari- 
zation. Sterzi (21) is unwilling to accept the interpretation of 
Retzius given to the eminentia saccularis, and believes that the 
saccus vasculosus has nothing in common with the latter either 
in position or structure, while Perna (22) on histological grounds 
“maintains that there can be no homology between the saccus 
vasculosus and the so-called ‘savcular eminence.’ | 
The post-infundibular eminence has been shown in illustration 
by many authors in a number of different species, although 
it has not always been definitely referred to in their text. Herrick 
and Obenchain (23) in their reconstruction of the brain of Ichthy- 
omyzon concolor figure a small, unleadered eminence ventrad to 
the mammillary body, in which there appears on mid-sagittal sec- 
tion a small recess communicating with the third ventricle. The 
only reference to this element made by these authors is the atten- 
tion which they call to the fact that the post-infundibular com- 
missure passes through the more cephalic portion of the structure. 
