236 FREDERICK TILNEY 
The subsequent history of the post-mammillary evagination 
shows that this recess is involved in the development of the cor- 
pus interpedunculare. In the infundibular region, the optico- 
infundibular ridge is much less prominent and the entire region 
is increased in size. Its notable characteristics at this stage 
are a large infundibular process (13) and a prominent ee 
infundibular eminence (34). 
The infundibular evagination now contains an extension of an 
prosencephalic ventricle, in this respect differing from the con- 
ditions in the 4.5 mm. embryo in which the infundibular process 
is solid and as yet contains no ventricular extension. 
The optic vesicle (29) presents a distinct cupping upon its 
latero-cephalic surface and is continuous with the lateral wall of 
the prosencephalon by means of a constricted stem, the optic 
peduncle (30). Along the cephalic surface of this peduncle 
runs a shallow groove, which becomes expanded as it passes 
out upon the latero-cephalic surface: of the cup. Entally a 
groove connects the canal of the two optic stems across the floor 
of the prosencephalon. This is the interoptic groove. Cephalad 
as well as caudad to this groove the flooris thin. In this stage, 
therefore, the remodelling of the floor has resulted in the sub- 
division of the mammillary region, forming the mammillary and 
post-mammillary evaginations, while laterally the anlages of 
the mammillary bodies have become defined. The infundibular 
region likewise shows an advance in its subdivisions, 1.e., the 
infundibular process (13) and the post-infundibular uminignee 
(34). The rest of the infundibular regions still remains in the 
general plane of the prosencephalic floor. 
Cat embryo of 10 mm.; Specimen No. 498 (fig. 9). The changes 
in this stage are more evident in the telencephalon and thala- 
mencephalon than in the floor-plate. The optic cup is now 
more pronounced than in the eae stage, the optic stem still 
more constricted. 
Entally a well marked interoptic groove (19) is present, and 
immediately caudal to this the floor-plate is thickened to form 
the interoptic torus. The floor of the infundibular region from 
the torus to the infundibular evagination is thin. The infundib- 
