1909-10.] Nervous Mechanism of Alimentary Canal of the Bird. 337 
remove part of the yolk, but in this case the embryo suffered so much that 
it was practically useless. 
Young plaice were procured from the same source, and when embedded 
in agar were found to cut easily, and reacted well to the silver-nitrate stain. 
For various reasons, however, this material was abandoned in favour of 
embryonic chicks. 
3. Results. 
The earliest age at which chick embryos were examined in connection 
with this investigation was at the end of the second day of incubation. 
At this age the rudimentary spinal cord appeared as a hollow rod of 
cells, which were roughly divisible into three layers according to their 
shape. The innermost layer was comprised of fairly circular cells ; the 
next of polygonal or pear-shaped cells ; while the outermost layer was 
formed by loosely arranged pyriform cells. All the cells showed well- 
marked nuclei. At the postero-lateral part of the spinal cord a few nerve 
fibres were recognised, which evidently came from the cells of the middle 
layer. The rudiments of the posterior spinal ganglia were seen at this 
stage as a proliferation of cells situated at the postero-lateral position of 
the spinal cord (PI. I. fig. 1). These cells were very similar to those 
comprising the two outer layers of the spinal cord, and, like them, showed 
well-marked nuclei. The cells in that part of a spinal ganglion nearest to 
the spinal cord were seen to be closely arranged, while between them lay a 
very few nerve fibres, evidently derived from the spinal cord. At the 
ventral extremity of a spinal ganglion the cells were, on the other hand, 
loosely arranged, while a very few were seen to have broken away from 
the ganglion, and lay among the mesoblastic cells at some little distance 
from the ganglion. These cells were distinguished from the mesoblasts by 
their size, which was somewhat larger, and by their pigmentation with the 
silver stain being darker. From the appearance and position of these 
nerve cells they seemed to form the beginning of a ventral-cell migration 
from the posterior spinal ganglia, similar to that described by His for the 
formation of the sympathetic chain. No trace of the anterior spinal nerve 
roots was seen at this stage, neither was any nerve structure found in the 
splanchnopleure. 
At the middle of the third day of incubation the central nervous 
system was found to have advanced considerably in development. The 
posterior spinal ganglia were larger, better stained, and showed the presence 
of more nerve fibres between the cells. At the position of the anterior 
spinal nerve roots a few very delicate and faintly stained nerve fibres were 
