234 FREDERICK TILNEY 
vesicle to the apex of the infundibular region. This is the 
optico-infundibular groove (31). It occupies relatively the same 
position as the horizontal limb of the primitive optic sulcus. 
Its increased prominence upon the ental surface of the ven- 
tricular wall appears to have occasioned a corresponding ridge 
upon the ectal surface, the optico-infundibular ridge. 
Fig. 7 Mesial view of forebrain reconstruction of a 4.5 mm. cat embryo 
(26 somites). X 150. The unshaded area shows the cut surfaces of the re- 
construction. 8, ectoptic zone of Schulte; 17, infundibular region; 18, infundib- 
ular evagination; 24, mid-brain; 25, mammillary evagination; 29, optic evagi- 
nation; 31, optico-infundibular groove; 36, post-infundibular eminence; 43, 
thalamencephalon; 44, telencephalon; 46, tubercle of the floor of Schulte. 
The mammillary region, although increased in size as com- 
pared with the earlier stages, manifests no other changes. Dorsal 
to it is the tuberculum postero-superius (45), while ventrad is 
