THE DIENCEPHALIC FLOOR Pail 
gives it all the characteristics of the mammalian and sauropsidan 
‘pars tuberalis. In the bird and mammal the caudal extremity 
of the post-chiasmatic eminence becomes constricted to form the 
infundibular stem (1/1), but in the selachian this constriction 
is less marked so that the stem is wide and short. It is difficult 
to appreciate it at all except by means of reconstruction. At 
its caudal extremity the stem becomes rapidly expanded to form 
the spacious infundibular process (13). This process presents 
two characteristic surfaces, i.e., a ventral one which is smooth 
and in contact with the pituitary gland, the pituitary surface: 
and a dorsal convoluted saccular surface, membranous in 
character and highly vascular. This latter forms the saccus 
vasculosus. : 
Dorsal to the saccus vasculosus is a small protuberance which 
differs structurally from the saccus in that it is composed chiefly 
of neural tissue. This is the post-infundibular eminence (34), 
dorsal to which is a larger protuberance, the posterior lobe (27). 
This structure forms a prominent eminence at the point of junc- 
ture between the mid-brain and the interbrain. Laterally its 
extremities project free of the adjacent neural tissue. These 
lateral extremities are in connection with a less protuberant 
median portion of the lobe. The transverse diameter of the 
lobus posterior is about twice that of the infundibular emi- 
nence; both protuberances are symmetrically disposed with 
reference to the mid-sagittal plane. Cephalad the recess of 
the posterior lobe is in direct communication with the dien- 
cephalic ventricle; caudad it opens into the post-infundibular 
recess (35). The recess of the infundibular process (40) is bounded 
ventrally by the pituitary surface of the infundibular process, 
while caudo-dorsally it is limited by the saccular surface forming 
the saccus vasculosus. This surface is thrown into a number 
of convoluted folds thus producing the diverticula sacci vasculosi 
(6). Ventral to the saccus vasculosus the infundibular recess 
presents a marked subdivision, the hypophyseal recess (10), 
while both the pituitary and saccular surfaces are so prolonged 
laterad as to form two long tapering processes, each of which 
contains a lateral extension of the infundibular recess. Corre- 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 25, NO. 3 
