THE DIENCEPHALIC FLOOR 243 
of the chiasmatic process appears to be due to the presence of 
increasing numbers of commissural fibers. The post-chiasmatic 
eminence (32) has increased in size, both in the median line as 
well as laterally, where it now begins to present free extremities. 
Its recess is deep and projects cephalad under the chiasmatic 
process in such a way that coronal sections of the brain in this 
stage show a distinct recess which extends forward beneath the 
chiasmatic process. The infundibular stem and the infundibular 
process have both increased in size; the tendency of the latter 
to extend its growth cephalad has about ceased, and the entire 
infundibular process seems to be on the point. of swinging dorso- 
caudad in its further development. The general shape of the 
process at this stage is oval, and the two relatively long lateral 
processes appear on either side. The infundibular canal (/2) 
is relatively longer and the recess of the infundibular process 
not only more spacious but more definitely demarcated from the 
canal than in any earlier stage. 
Caudad to the infundibular stem (1/1) 1s a large post-infundib- 
ular eminence (34) containing a well defined post-infundibular 
' recess (35), which latter is separated by a transverse ridge from 
the mammillary recess. The neural wall bounding this recess 
and thus forming the mammillary eminence (27) has notably 
increased in thickness, so that the relative dimensions of the 
diencephalic ventricle are being lessened by the encroachment 
due to the thickening of the brain floor in the region of the 
mammillary bodies. This thickening particularly affects the two 
lateral mammillary diverticula, while the median mammillary 
and post-mammillary evaginations are no longer discernible. 
Thickening has also occurred in the region of the post-mammil- 
lary evagination, and this area now shows an ectal protuberance 
marking the site of the corpus interpedunculare (4). 
Cat embryo of 51 mm.; Specimen No. 104 (fig. 13). The changes 
observed in this stage involve the further development of the 
chief tendencies observed in the 30 mm. embryo, 1.e., the pro- 
nounced caudal deflection of the infundibular process (13) which 
appears as a distinct appendage to the floor of the ventricle, due 
to the increased length and constriction of the infundibular stem 
(11). This process now presents two surfaces, one which faces 
