248 FREDERICK TILNEY 
of the primitive optic vesicles (29) and the formation of the 
ectoptic zone (8) which now presents its dorsal, cephalic and 
ventral segments. Further advance is found in the formation 
of a definite mammillary region (25). The boundary between 
midbrain and forebrain is indicated by the tuberculum postero- 
superius (45). The ventral segment of the ectoptic zone appears 
as a well defined infundibular region (1/7) and a wide, shallow 
groove extends from the orifice of the optic evagination to the 
apex of this region, forming the optico-infundibular groove (37). 
A slight transverse ridge separates the mammillary and infundib- 
ular regions thus marking the position of the tubercle of the floor 
Fig. 17 Mesial view of forebrain reconstruction of 6.75 mm. chick.  X 150. 
17, infundibular region; 20, lamina terminalis; 24, mid-brain; 25, mammillary 
region; 29, optic vesicle or evagination; 31, optico infundibular groove; 44, 
ee eee 48, tuberculum postero-superius; 46, tubercle of the floor of 
Schulte. 
