256 FREDERICK TILNEY 
apex of the optic vesicle is a shallow groove which traverses the 
tube transversely and demarcates the vesicle from a prominent 
eminence, the mammillary region (25). The ental surface of 
this model (fig. 22) shows the optic vesicle as a shallow evagi- 
nation from the deepest portion of which a sulcus may be traced 
ventrad and caudad to the tubercle of the floor (46). This is 
the optic sulcus. The ectoptic zone has not yet made its ap- 
pearance. The mammillary region is well defined both as an 
ectal protuberance and an ental recess. The prosencephalon 
thus presents the two primitive constituents which are char- 
acteristic of this period in the cat and chick. 
Fig. 23 Eetal view of forebrain shown in figure 22. X 150. The unshaded 
area shows the cut surfaces of the reconstruction., 17, infundibular region; 
25, mammillary region; 28, neuropore; 29, optic evagination. 
Mustelus embryo of 7 mm.; Specomen No. 294 (fig. 24). In 
this embryo the neuropore is still open, while the advance in 
development is indicated by the marked reduction of the optic 
vesicle (29) and the appearance of an ectoptic zone (8) presenting 
the three characteristic segments. The dorsal and cephalic 
segments present respectively the anlages of the thalamen- 
cephalon (43) and telencephalon (44), while the ventral segment 
now appears in the form of a definite infundibular region (17). 
The reduction of the optic vesicle has been carried to such a 
degree that the optic cup and optic peduncle may both be dis- 
