THE DIENCEPHALIC FLOOR 233 
tive optic vesicles early become profoundly remodelled giving 
rise to a much reduced optic evagination and a pronounced area 
of the neural wall which surrounds it. This area is called the 
ectoptic zone. It presents itself as an arc of three distinct seg- 
ments, the dorsal segment giving rise to the thalamencephalon, 
the cephalic segment to the telencephalon and the ventral 
segment to the infundibular region. All of these secondary 
derivatives are present in the cat embryo of twenty-one somites 
(see fig. xxxvili in loc. cit.). 
Development of the diencephalic floor of the cat 
Cat embryo of 4.5 mm.; twenty-six somites; Specimen No. 495 
(fig. 7). The forebrain of this embryo shows an advance over 
the conditions observed in the embryo of twenty-one somites. 
All of the primitive elements of the prosencephalon previously 
described may be recognized. The optic vesicles (29) ‘are 
further reduced in size, and present a constriction at their point 
of attachment to the neural tube. Their external configuration 
is still convex upon all surfaces. The ectoptic zone shows its 
characteristic division into thalamencephalon (43), telencephalon 
(44) and infundibular region (1/7). The regio mammillaris 
(25) is well marked and ectally separated from the apex of the 
infundibular region by a shallow transverse furrow, the tubercle 
of the floor (46). 
The most pronounced changes are evident in the regio infun- 
dibularis, not only in the fact that this region is enlarged but also 
because it presents two subdivisions, both evolved from the apex 
of this area. The first of these subdivisions appears as a ventral 
protrusion which is conical in shape, the infundibular evagi- 
nation (18). Dorsal and caudal to this appears a large, shallow 
diverticulum which, because of its position, may be designated 
the post-infundibular evagination (36). The remainder of the 
infundibular region forms the greater part of the floor of the 
prosencephalic ventrigle. Its general plane of inclination is 
caudo-ventral, and its thickness is uniform throughout its entire 
extent. Entally it presents on either side a deep groove passing 
along the lateral wall from the now circular orifice of the optic 
