246 FREDERICK TILNEY 
ultimate plane of the floor as yet, for the region of the post- 
infundibular and mammillary areas has assumed a more vertical 
position than is true of the 15, 30 and 51 mm. stages and this 
position will later be so modified in attaining adult conditions 
that the most dorsal element in this region, the corpus inter- 
pedunculare (4), will be rotated ventrad through an arc of nearly 
90°. All of the eminences observed in the early stages and their 
corresponding recesses are here present. The infundibular proc- 
ess (13) presents two well marked, lateral processes, one on 
either side; the post-chiasmatic eminence (32) likewise presents 
two lateral processes which project free beneath the supra- 
jacent lateral eminences of the tuber cinereum. The wall of 
the post-infundibular eminence (34) has increased in thickness 
and it bounds a spacious post-infundibular recess (35). Two 
mammillary bodies are now present, but the mammillary recess 
(26) cannot be defined because of the thickening which has 
progressed in the development of the mammillary region. The 
corpus interpedunculare (5) forms the most dorsal element in 
. this portion of the ventricular floor; the prechiasmatic and 
supraoptic recesses (38 and 42), the latter extending out upon 
the optic nerve for some distance, are both present. 
Development of the diencephalic floor in the chick 
Chick of twenty-three hours; eight somites; Specumen No. 618 
(fig. 15). The forebrain at this stage consists exclusively of 
the large optic vesicles similar in all respects to the vesicles in 
the cat, although their transverse diameter is greater and their 
altitude less. Entally the horizontal segment of the optic sulcus 
is well defined, extending from the deepest portion of the optic 
evagination (29) obliquely meso-caudad and converging with 
the corresponding sulcus of the opposite side. The angle formed 
by the convergence of these two sulci is occupied by a clearly 
defined prominence, the tubercle of the floor (46). The neuro- 
pore (28) is still open for a considerable distance at the cephalic 
extremity of neural folds, but its closure is more advanced than 
in the cat embryo of eight somites. 
Chick of forty-nine hours; twenty somites; Specimen No. 619 
(fig. 16). The advances in this stage consist in the. reduction 
