THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 43 
and conspicuous demarcation exists, therefore, between the two layers 
from the very beginning, and it is easy to trace the subsequent fate 
of the two layers owing to this difference of pigmentation. The pig- 
mented cells form the lining cells of the central canal, and becoming 
elongated, stretch out between the cells of the nervous layer; while 
the latter, on their side, invade and press between the pigmented 
cells. In this case, owing to the pigmentation of the epithelial layer, 
embryology points out in the clearest possible manner how the 
central nervous system of the vertebrate is composed of two struc- 
tures—an epithelial non-nervous tube, on the outside of which the 
central nervous system was originally grouped; how, as develop- 
ment proceeds, the elements of these two structures invade each 
other, until at last they become so involved together as to give rise 
to the conception that we are dealing with one single nerve tube. 
It is impossible for embryology to give a clearer clue to the past 
history than it does in this case, for it actually shows, step by step, 
how the amalgamation between the central nervous system and the 
old alimentary canal took place. 
Further, consider the shape of the tube when it is first formed, 
how extraordinary and significant that is. It consists of a simple 
dilated anterior end leading into a straight tube, the lumen of which 
is much larger than that of the ultimate spinal canal, and terminates 
by way of the neurenteric canal in the anus. 
Why should the tube take this peculiar shape at its first forma- 
tion? No explanation is given or suggested in any text-book of 
embryology, and yet it is so natural, so simple: it is simply the shape 
of the invertebrate alimentary canal with its cephalic stomach and 
straight intestine ending in the anus. Again embryology indicates 
most unmistakably the past history of the race. How are the 
nervous elements grouped round this tube when it is first formed ? 
Here embryology shows that a striking difference exists between the 
part of the tube which forms the spinal cord and the dilated cephalic 
part. Fig. 21, A (2), represents the relation between the nervous 
masses and the epithelial tube in the first instance. At this stage 
the nervous material in the spinal cord lies laterally and ventrally 
to this tube, and at a very early stage the white anterior commissure 
is formed, joining together these two lateral masses; as yet there is 
no sign of any posterior fissure, the tube with its open lumen extends 
right to the dorsal surface. 
