250) FREDERICK TILNEY 
reduction of the optic vesicles, there are no notable changes in 
this stage. 
Chick of five days, twenty hours; Specimen No. 326 (fig. 18). 
In this stage a marked change has occurred in the infundibular 
region (17) namely, the appearance of two distinct evaginations 
at the apex of this region, the more ventral of which is involved 
in the formation of the infundibular process (13), while the more 
dorsal one ultimately forms the post-infundibular eminence 
(34). These two evaginations of the infundibular region are . 
more pronounced in the chick than in either the cat or the dog- 
fish. The mammillary region (25) has increased in size, but 
occupies a position dorsal to the infundibular region as in the 
immediately preceding stage. The mammillary recess (26) 
is correspondingly larger. Thus the floor of the third ventricle 
now presents three separate evaginations, the infundibular 
evagination (18), the post-infundibular evagination (36) and 
the mammillary evagination (26), a condition corresponding 
in all details to the early history of the development in the 
.mammillary and infundibular regions of the cat. At this stage 
also a large chiasmatic process (2) has appeared thus demar- 
cating a prechiasmatic recess (38) and a post-chiasmatic recess 
(33). This latter recess is marked upon the exterior surface by 
a prominent post-chiasmatic eminence (32). 
Chick of eight days; Specimen No. 316 (fig. 19). In this stage 
all of the definitive elements of the diencephalic floor are present. 
The chiasm (4) and the chiasmatic process (2) have increased 
in size with the consequence that the prechiasmatic and post- 
chiasmatic recesses (38 and 33) are more pronounced. The 
supraoptic crest (47) and supraoptic recess (42) are both present. 
The post-chiasmatic eminence (32) has also increased in promi- 
nence. It now shows a distinct longitudinal furrow which 
marks the inception of the median post-chiasmatic groove. In 
the region of this groove the floor is relatively thin, while upon 
either side of it the neural tissue has a considerable thickness. 
In the caudal portion of the infundibular region the dorsal and 
ventral evaginations are more marked than in the next earlier 
stage and in them may be distinguished the anlages of the in- 
fundibular process (13) and post-infundibular eminence (34). 
