238 FREDERICK TILNEY 
relation to the infundibular process and for this reason may be 
designated the post-infundibular eminence (34). The cavity 
of this eminence which communicates with the third ventricle 
forms the post-infundibular recess (35). A slight ridge which 
is the remnant of the floor tubercle (46) (tuberculum postero- 
inferius) separates the post-infundibular recess from the mammil- 
lary evagination (26) which, as in the 7 mm. embryo, presents 
two subdivisions affecting the mid-sagittal plane, ie., the 
mammillary and post-mammillary evaginations; while laterally 
two large diverticula defining the anlages of the mammillary 
body have increased in prominence but still retain an ample 
recess, the recessus mammillaris, which is in communication 
with the third ventricle. Dorsal to the post-mammillary 
evagination is the tuberculum postero-superius (45) now some- 
what increased in size. 
Cat embryo of 12 mm.; Specimen No. 217 (fig. 10). The 
advance in this embryo over that of 10 mm. appears in the fact 
that all the definitive elements of the diencephalic floor are now 
discernible. The most conspicuous changes affect the region 
of the interoptic groove and the area caudal to it. Where this 
optic groove formerly appeared as a furrow extending between 
the orifices of the optic peduncles, the floor is still relatively 
thin; but caudal to this groove, both entally and ectally, it 
presents a pronounced thickening, the ectal increase in size being 
due to the beginning formation of the optic chiasm (4), while 
entally the thickening forms the chiasmatic process (2). Thus 
the furrow in front of the process becomes the prechiasmatic 
recess (38). Ascending from the latter the lamina terminalis 
(20) extends obliquely cephalo-dorsad to join the roof-plate. 
Quite as notable as the changes which have occurred in the 
region of the chiasm are those which appear in the area immedi- 
ately caudad to it. Here the diencephalic floor, without increas- 
ing in thickness, presents a ventral protrusion which forms the 
post-chiasmatic eminence (32). This eminence is symmetrically 
disposed with reference to the mid-sagittal line. Caudal to this 
is the infundibular stem (//) considerably lengthened and ex- 
panding to form the infundibular process (13). Between the 
