DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 513 
is—=7,'9);,6; 106; 7,6, 4, 3,115 6; 55-574, 55:3, 6, 23 7, 7, 6; 8, 
7, 4,6. The sections in the two series compared are of the 
same thickness, although it takes twenty-nine sections to pass 
through a segment in the younger embryo, which twenty-five 
sections cover in the older embryo. For the second segment, 
I found above the level of the motor nerve 138 cells, an 
increase of only twelve cells. The third segment gave 137 
cells, an increase of fourteen cells. In the fourth segment I 
found 186 cells, showing rapid increase, probably due, as above 
mentioned, to the formation of a large brachial ganglion, 
although as yet no grouping of the cells into a ganglionic 
mass is found. 
These figures are merely of relative value, as the number 
of neural crest cells varies in different embryos at about the 
same stage of development, and varies also on opposite sides of 
the same embryo. 
Comparing the number of cells in the first segment of the 
two series, we find that the neural cells of the younger embryo, 
even had there been no increase by division, which is impro- 
bable, more than suffice to account for all of the connective 
tissue above the level of the motor nerve-root in the older 
embryo. I have not extended the enumeration, as the figures 
given demonstrate the continuity and extent of the tissue of 
ectodermic origin at the side of the brain in segments in which 
either no spinal ganglion or but a small one is formed. Part 
of the neural cells, however, that lie in the posterior division of 
the vagus segment form the posterior vagus root, the two 
vagus roots thus corresponding to the divisions of the primi- 
tive segment. 
There is not only a rapid multiplication of neural cells to 
form the spinal ganglia, but also an additional migration of 
cells from the spinal cord through the dorsal nerve-root. The 
neural crest cells of the trunk resemble those of the head in 
having at first no special connection with one another, or with 
the central nervous system, and only on reaching the level at 
which the spinal ganglion is to develop do they send prolonga- 
tions in two directions, one constituting the peripheral nerve, 
