1909-10.] Nervous Mechanism of Alimentary Canal of the Bird. 345 
cellular chains (Pl. II. fig. 3, PI. III. fig. 4). In several chicks at the sixth 
day of incubation the arrangement, elongation, and union of the nerve cells 
of the primary nerve-cell chains in the mesentery suggest that some at 
least of the nerve fibres which appear in the mesentery about this time 
are formed by the union of these cells (PI. III. fig. 4). In many 
sections, on the other hand, this appearance is not found, but in those 
cases the nerve fibres are not observed to be in direct relationship with 
cells in the spinal cord. 
Several difficulties seem to me to oppose the complete acceptance of the 
outgrowth theory for these autonomic nerves as fibres. Firstly, the fact that 
at the fifth and sixth days well-marked chains of nerve cells are found in 
the mesentery, while at the seventh they are almost, if not wholly, replaced 
by nerve fibres (contrast PI. II. fig. 1, PI. II. fig. 3, and PI. III. fig. 4), 
seems to me to offer a problem as to the fate of the nerve cells form- 
ing those chains. So many may be utilised for ganglia and plexuses 
in the gut wall, but the increase of the plexiform nerve mechanism in this 
latter position between the sixth and seventh days (PI. IV. fig. 1 and PI. 
IV. fig. 3) is not, in my opinion, sufficient to account for all the nerve cells. 
Secondly, if the nerve fibres are in reality outgrowths from the spinal cord, 
their rate of growth must be exceedingly rapid, for, as shown by PI. II. 
fig. 3, they reach the gut at the seventh day, their first appearance in the 
mesentery being noted at the sixth day. Should they, therefore, be 
outgrowths, they form in a little over twelve hours a connection between 
the spinal cord and the gut. The migration of the nerve cells from the 
spinal cord to the gut was much slower, for they were found behind the 
aorta and in the region of the notochord at the end of the third day, 
while it was not until four and a half days that the gut wall was reached. 
Considering the increased density of the embryonic tissue at the seventh 
day contrasted with that on the third and fourth days, and the longer 
distance to be traversed by an outgrowth from the spinal cord to the gut, 
it seems improbable that the nerve fibres seen in the mesentery at the 
seventh day are outgrowths from cells in the spinal cord. I am inclined to 
attribute the appearances presented in sections of chicks at six and seven 
days’ development, in which the mesenteric nerve fibres seem to be developed 
independently of the cellular nerve chains, to the fact that I have failed to 
hit the correct time at which the transformation of the cellular nerve 
chains to nerve fibres occurs. This point of time, I consider, I had the good 
fortune to hit in several cases (PI. III. fig. 4), and in those cases I found 
an appearance, as seen in the figure referred to, of a “ beaded ” nerve fibre, 
each “ bead ” being formed by a nerve cell. 
