11 F, L. LANDACRE 
In the closure of the neural groove (fig. 2) the superficial 
pigmented cells lining the dorsal two-thirds of the neural tube 
come into contact and obliterate that portion of the canal lined 
by flat cells. The line of juncture is indicated by the heavy 
pigmentation of the cells. The dorsomesial portion of the wall 
of the tube has a looser texture and less of a syncytial character 
than the lateral and ventral portions. The neural tube as a 
whole is well delimited from the ectoderm except at the dorsal 
border. 
The outline of the neural tube (fig. 3) as distinct from the 
neural crest first becomes apparent when the cells of the tube 
assume a syncytial character with their nuclei arranged with 
their long axes toward the center of the neural canal. At the 
same time the neural crest while still forming a conspicuous 
wedge in the dorsal portion of the tube is evidently now largely 
outside the limits of the tube. The dorsal third of the tube 
becomes neural crest and presents the appearance of being 
erupted from the tube. The tube is horseshoe-shaped with the 
open dorsal portion filled with a wedge of neural-crest cells. 
The tube except at the ventral border is of uniform thickness. 
The next step (fig. 4) involves the further exclusion of neural- 
crest cells from the dorsal wall of the tube. In this process many 
of the loosely arranged, heavily pigmented cells are left for a 
time in the position of the original wedge. In fact, after the 
neural crest is well defined and has begun to migrate ventrally 
(fig. 5) a few cells of this type still form the roof plate of the 
neural canal. In this stage (fig. 5) the greater portion of the - 
neural crest rests upon the dorsal portion of the neural tube, 
but there are two prominent lateral extensions lying between 
the dorsolateral border of the neural tube and the ectoderm. 
At a slightly later stage (fig. 6) the neural crest is represented 
almost exclusively by these lateral extensions. The large mass 
of cells previously lying over the tube is now represented by a 
few flat cells connecting the two lateral portions of the neural 
crest. These flat cells disappear later. In figure 6 the original 
wedge seems to be represented by a few irregularly arranged 
cells. 
