OTIC AND OPTIC PRIMORDIA IN MAN PALE 
in some sections there is a corresponding sulcus dorsally, i.e., 
on the future ventricular surface. On the left side three such 
thickenings can be recognized, one near the rostral end of the 
folds, one in the region of the cranial flexure, that is, in the mid- 
brain, and a third in the first hindbrain segment (cf. p. 218 infra). 
A few sections through the latter level show a further differentia- 
tion in that the thickened neural fold has no lower (external) 
boundary. It seems to merge with the mesenchyme beneath. 
As the later stages show, this is the very beginning of the neural- 
crest proliferation; in other words, there is an active migration 
of cells from the neural folds. The histologic picture in these 
particular sections is not above reproach, but under low powers 
of the microscope it presents the same appearance as do the 
perfectly preserved embryos of this period. These specimens 
make it clear also that we are not dealing in this case with an 
artefact. It is significant that the proliferation begins in the 
region where the semilunar ganglion can subsequently be 
identified. 
In H87 (eight somites) it has been possible to study these 
primordia carefully, as the plane of section is particularly favor- 
able. The forebrain here is in the form of a broad neural plate 
(fig. 3) which is in marked contrast with the domed folds of 
the four-somite H279, the Pfannenstiel-Krémer embryo (‘Klb’) 
and the eight-somite Dandy embryo. On the other hand, it 
resembles several embryos in this respect, e.g., the three-somite 
Carnegie No. 1878 (Ingalls), Wilson’s ‘H 98’ (nine somites), H637 
(eleven somites), the twelve-somite H 197, and the fourteen- 
somite N. Y. U. specimen (Wallin). 
From the levels which show the pouch of Rathke [?] (fig. 4, 
Rk. p.) there are lateral thickenings of the folds extending caudally 
into the hindbrain, as is indicated by the stippling in figures 3 
and 4 (op. cr. prim. and cr.). This longitudinal ridge may be 
divided into two parts, a larger broader area which converges 
rostrally toward its mate of the other side and a more caudal 
one which is narrower and more laterally situated. The con- 
striction between them is in the region of the midbrain. This 
primordium gives rise not only to the optic vesicle, but to neural 
