OTIC AND OPTIC PRIMORDIA IN MAN 223 
the future lens epithelium. ‘This is true of two embryos in the 
Carnegie Collection, No. 12 (fourteen somites) and No. 470 
(sixteen somites). In the latter, as figure 9f shows, we have a 
fully formed optic vesicle resembling that of the His embryo 
‘EB’ (04) and of the twins described by Watts (15). It is 
younger in the fourteen-somite embryo H8 in which the lumen 
is narrower and there is no ectodermal contact, although the 
anterior neuropore is as small as in the sixteen-somite No. 470. 
Discussion—The optic vesicle 
If we compare e and f of figure 9, it is clear that the optic 
anlage has begun to balloon out, by an interstitial growth as 
well as by a thinning of the wall. The optic sulcus has become 
V-shaped so that the anlage is a trough about as long at the 
base as it is deep. This holds not only for the fourteen-somite 
HS8 and the sixteen-somite <470 (Carnegie coll.), but also for the 
embryos of Watts and that described by Bremer (’06). It is not 
until the twenty-three-somite stages that the vesicle has assumed 
a more or less spherical shape. It is difficult as yet to say how 
much of the area indicated by the solid color in figure 9e enters 
into the formation of the definitive optic vesicle. The narrow 
band of neural epithelium which separates vesicle and head 
ectoderm appears to grow very rapidly as the optic sulcus widens 
and deepens, and this growth certainly plays a part in the approxi- 
mation of the neural folds. After their closure this lateralmost 
part of the original neural plate constitutes all there is of brain 
wall separating the two vesicles dorsally. It is of course possible 
that all of the dorsal and lateral diencephalic wall which we find 
between the optic stalks in later stages is derived from it. On the 
other hand, it seems more probable that, as the vesicle gradually 
pinches off, some of the original evagination is incorporated into 
the brain wall both dorsal as well as rostral and caudal to the 
developing optic stalk. Perhaps it would be better to say that a 
portion of the lateral brain wall is at first dragged out with the 
optic evagination. This would hold particularly for the zone 
between vesicle and head ectoderm. Schulte and Tilney (15) 
