268 FREDERICK TILNEY 
thalamencephalon takes origin in the dorsal segment. From 
these facts it seems fair to assume that the third element of this 
zone, namely the ventral segment, possesses similar possibilities 
of development and the three segments of the ectoptic zone may 
therefore be regarded as dynamically codrdinate. The attempt 
to substantiate this assumption is given in discussing the de- 
rivatives of the infundibular region. 
The optic vesicles. In the period of development shortly after 
the closure of the neuropore the floor of the forebrain consists 
of the lamina terminalis, the optic evaginations and the infundib- 
ular and mammillary regions. In sauropsid and mammalian 
forms the lamina terminalis assumes a vertical position in the 
_ later stages of development; in the ichthyopsid, on the contrary, 
it remains horizontal and is thus an element in the floor of the - 
ventricle cephalad to the optic chiasm. Entally the evaginations 
of the primitive optic vesicles are indicated by the optic sulcus 
which does not in any way correspond to the preoptic and post- 
optic grooves of authors. This sulcus is bilateral. It does not 
- cross the median line as the above mentioned grooves are shown 
to do, and in its early appearance it presents itself as an arcuate 
fissure consisting of a vertical and horizontal segment, the latter 
extending caudad to the tubercle of the floor, while the convexity 
of the entire arc is directed cephalad. Subsequently, when the 
primitive optic vesicle has become reduced by the remodelling 
which results in the formation of the ectoptic zone, the vertical 
segment of the optic sulcus disappears but the position of its 
horizontal segment is occupied by the optico-infundibular groove. 
At this stage the optic vesicle has so changed its external con- 
formation as to have the appearance of a pedunculated divertic- 
ulum of the forebrain to which latter it is attached by a con- 
stricted, hollow stalk, the optic peduncle. In relatively late 
stages after the appearance of the chiasmatic process a transverse 
eroove connects the orifices of the optic peduncles. A groove 
is also formed caudad to the chiasmatic process. These undoubt- 
edly correspond to the post-optic and preoptic grooves already 
mentioned, but it will be noticed that their appearance develop- 
mentally is relatively late. The proximal portion of the optic 
