THE DIENCEPHALIC FLOOR Ae 
lateral eminences of the tuber is not clear in the light of the 
material studied. Conditions in the teleost, however, in which 
the hypoarium presents small lobi inferiores and much larger 
lobi laterales, may be regarded as suggestive, since the progressive 
reduction of the inferior lobes has been shown to result in the 
formation of the post-chiasmatic eminence and a similar diminu- 
tion of the lateral lobes might, therefore, determine the emi- 
nentiae laterales hypencephali. This homology is tentatively 
offered, since it requires further proof in the development of 
the teleost to establish it. 
The mammillary region. * The posterior lobe is the most caudal 
structure in the diencephalic floor of the selachian. Its relations 
have already been described (page 231). It consists of a median 
portion and two lateral processes which project free, one upon 
either side. Its characteristic feature appears in the fact that 
it contains a large recess of the third ventricle which extends 
from the median portion into the two lateral processes. Sterzi 
(21) has figured and described the posterior lobe in selachians; 
von Kuppfer (7) has shown it in Bellostoma, Squalus acanthtus, 
and Necturus. Edinger (18) describes it in the selachian as 
the lobus posterior sive saccus. infundibuli. Johnston (11) 
shows the posterior lobe as well as the post-infundibular eminence 
in a figure of the mesial surface of the right half of the brain in 
Squalus acanthius, although neither of these structures is spe- 
cifically named by him in this place. Herrick and Obenchain 
(23), in their illustration taken from the reconstruction of the 
brain in Ichthyomyzon concolor, indicate a structure similar 
in relations and characteristics to the posterior lobe which they 
call the corpus mammillare. In several respects the designation 
given the structure by the last named authors seems to be most 
in keeping with the facts, for although the majority of investi- 
gators have employed the term posterior lobe in selachian and 
teleosts, the embryological history of the structure clearly shows 
that it is derived from the primitive mammillary region. In 
the ichthyopsid the mammillary region develops in such a way 
as to form a posterior lobe presenting the characteristics already 
described and retaining a recess accessory to the third ventricle, 
