THE DIENCEPHALIC FLOOR 265 
process which, because of its close relation to the pituitary gland, 
is called the recessus hypophyseus (10). The pituitary and 
saccular surfaces of the infundibular process are continued laterad 
for a considerable distance forming two tapering extensions, the 
lateral processes of the processus infundibuli. During this 
development the dorsal infundibular evagination (36) manifests 
but little change; when, however, the infundibular process is 
formed the dorsal evagination, because of its position, becomes 
the post-infundibular eminence. 
3 
Fig. 30 Mesial view of brain reconstruction of 300 mm. Mustelus. X 25. 
The unshaded area shows the cut surfaces of the reconstruction. 2, chiasmatic 
process; 3, cerebellum; 4, chiasm; 7, epiphysis; 10, hypophyseal recess; 13, 
infundibular process; 14, infundibular process, saccular surface; 14, infundibular 
process, pituitary surface; 24, mid-brain; 27, mammillary body (posterior lobe); 
32, post-chiasmatic eminence (inferior lobe); 33, post-chiasmatic recess (recess 
of inferior lobe); 36, post-infundibular evagination; 39, paraphysis; 40, recess 
of the infundibular process; 4/, supra-optie crest; 42, supra-optic recess; 44, 
telencephalon; 47, velum transversum. 
The ontogenesis of the diencephalic floor in Mustelus, as in 
the cat and chick, may thus be traced from two primitive fore- 
brain elements, the optic vesicle and the mammillary region. 
From the optic vesicle is derived the ventral segment of the ectop- 
tic zone which gives rise to the infundibular region. From the 
optic vesicle the following structures are derived, i.e., the retina, 
