FATE OF NEURAL CREST IN HEAD OF URODELES 13 
Up to the last stage described there is practically no indication 
that cells are added to the crest, as defined, from the lateral 
ectoderm. The crest is continuous on its dorsal border with 
the inner layer of the ectoderm, and undoubtedly receives cells 
from this source (figs. 1, 2, 3, and 5), and in several series at the 
age of that from which figure 3 is taken and the crest is continuous 
ventrally with the inner layer (fig. 3). This continuity is usually 
absent at the level under consideration as the lateral portions of 
the crest become better defined (figs. 5 and 6) and also in the 
earlier stages (fig. 4). 
These conditions as described at the level of the VII sun 
are duplicated at the levels of ganglia V, IX, and X. In the 
intervals between these ganglia the neural-crest cells are less 
numerous and the lateral extensions contain fewer cells and do 
not reach so far ventrally. At the stage from which figure 5 
was taken (fig. 7), the neural crest is continuous throughout the 
whole region. In the stage from which figure 6 was taken 
(fig. 8) the crest is interrupted, except for a few scattered cells 
on the dorsal portion of the cord, between the V and VII ganglia 
and again between the VII and IX ganglia. 
THE MIGRATION OF THE NEURAL CREST AND ITS RELATION TO 
THE MESODERM 
The accuracy with which the migration of the neural crest 
ean be followed depends upon histological differences between 
cells derived from the ectoderm as distinguished from those 
derived from the endoderm. ‘These distinctions have been stated 
already in the introduction (p. 2). They will be assumed 
for the present and the migration of the crest will be described 
from the reconstructions given in figures 7 to 11. The discussion 
of the basis of these distinctions together with the discussion of 
two other disputed points, namely, the question as to the con- 
tribution by the lateral ectoderm of cells to the mesenchyme 
and the question as to the fate of cells in the anterior head region, 
will be deferred to a later section. 
As to the use of terms, Miss Platt rejected the term ‘mesento- 
derm’ presumably because the term implies a derm or germ layer 
