218 G. W. BARTELMEZ 
crest as well; that is, to ganglion cells and head mesenchyme. 
Hence the somewhat labored term ‘optic-crest primordium.’ It 
is a continuous zone on the left side of the H87; on the right 
it is difficult to be certain of it in two adjacent sections through 
the midbrain, a distance of 16 uw. Brachet (’06, p. 244), in his 
amphibian studies, has pointed out the interesting and perhaps 
significant fact that the cranial neural crest and retina arise from 
homologous regions of the neural folds, and the continuity of 
the primordium in this series adds interest to our speculations on 
the subject. 
As the figures show, the large rostral part of the primordium 
has a shallow sulcus on the upper, 1.e., ventricular, surface which 
marks the position of the future optic evagination and is, in fact, 
the optic suleus. As we approach the midbrain the primordium 
becomes narrower and involves the lateral part of the neural fold, 
and there is no sulcus (fig. 3). On the left at least the primordium 
continues into the hindbrain where cells are migrating out from it. 
Here, then, we have a typical neural crest. Some of these cells 
remain relatively closely packed and enter into the formation of 
the semilunar ganglion. Others become head mesenchyme. 
It is worthy of note, in view of the recent work of Landacre 
(21) ,that promptly after the proliferation begins the mandibular 
and hyoid arches begin to take form (ef. the Ziegler model of 
Eternod’s embryo ‘Du Ga’). 
The appearance of a section through the rostral area of the 
primordium is shown in figure 9a, where both optic sulci appear. 
The primordium is indicated by the solid color. It is improbable 
that all of this eventually enters into the optic vesicle, as we 
shall see below. 
It will be best to turn now to the eighth member of the series, 
the twelve-somite embryo H 197, because the plane of section 
in the forebrain is very like that of H87, whereas in the three 
intermediate stages it is less favorable. There is a great dorsal 
flexure in H 197 like that in the Wilson embryo H98 (714) and 
consequently it was possible to obtain sections perpendicular 
rather than tangential to the surface of the forebrain folds. 
The optic-crest primordium shows several changes. Its rostral 
