240 FREDERICK TILNEY 
infundibular stem and the mammillary evagination (26) is a 
small but distinct diverticulum of the floor appearing as a promi- 
nence upon the ectal surface and forming the post-infundibular 
eminence (34). 
The mammillary evagination is still large and its cavity 
spacious. In it may still be recognized the subdivisions already 
described, i.e., the two lateral evaginations of the mammillary 
bodies, the median mammillary evagination (26) and the post- 
mammillary evagination (37). All of the eminences mentioned 
have their corresponding recesses, i.e., the post-chiasmatic 
recess (33), the post-infundibular recess (35), the mammillary 
recess (26) and the post-mammillary recess. The ventricle 
extends through a short and narrow infundibular canal (12) 
into a relatively large recess of the infundibular process. 
The eye-cup in this stage is now completely formed. It is 
attached to the prosencephalon by the optic peduncle; but 
certain changes have occurred in this peduncle which have 
important bearings upon the structures evolved from it. As 
- the peduncle approaches the brain-wall it becomes rapidly 
expanded in the form of a distinct evagination of the prosenceph- 
alon, into which extends an expansion of the ventricle. Ventral 
to this evagination the optic peduncle has increased in thickness, 
due to the appearance of fibers forming the optic nerve and 
entering the optic chiasm (4). In this manner a diverticulum of 
the third ventricle comes to overlie the lateral portion of the 
optic chiasm and the proximal portion of the optic nerve. This 
diverticulum is the supraoptic recess (42). 
Cat embryo of 15 mm.; Specimen No. 505 (fig. 11). In this 
embryo a foreshortening has occurred in the diencephalic floor. 
This is due principally to the change in the inclination of the 
lamina terminalis (20) which is now vertical. In consequence 
of this alteration the prechiasmatic recess has become the most 
cephalic portion of the ventricle. The chiasmatic process (2) 
and the chiasm have increased in size. The post-chiasmatic 
eminence is still further expanded and its recess is larger. The: 
post-infundibular eminence (34) occupies a typical position be- 
tween the infundibular stem (1/7) and the mammillary evagi- 
