THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 39 
material throughout, and for some reason parts of it have lost their 
nervous function and thinned down; or else these thin epithelial 
parts are on their way to become nervous material, are still in an 
embryonic condition, and are of the nature of epiblast-epithelium, 
from which the central nervous system originally arose. 
The first explanation is said to be supported by embryology, for 
at first the nerve-tube is formed in a uniform manner, and then 
later, parts of the roof appear to thin out and so form the thin epi- 
thelial parts. If this were the right explanation, then it ought to 
be found that in the lowest vertebrates there is greater evidence of 
a uniformly nervous tube than in the higher members of the group: 
while conversely, if, on the contrary, as we descend the vertebrate 
phylum, it is found that more and more of the tube presents the 
appearance of a single layer of epithelium, and the nervous material 
is limited more and more to certain parts of that tube, then the 
evidence is strong that the tubular character of the central nervous 
system is not due to an original nervous tube, but to a non-nervous 
epithelial tube with which the original nervous system has become 
closely connected. 
The comparison of the brain region of the different groups of 
vertebrates (Fig. 19) is most instructive, for it demonstrates in the 
most conclusive manner how the roof of the nervous tube in that 
region loses more and more its nervous character, and takes on the 
appearance of a simple epithelial tube, as we descend lower and 
lower; until at last, in the brain of Ammoceetes, as represented in 
the figures, the whole of the brain-roof, from the region of the 
pineal eye to the commencement of the spinal cord, is composed of 
fold upon fold of a thin epithelial membrane forming an epithelial 
bag, which is constricted in only one place, where the fourth cranial 
nerve crosses over it. 
Further, the brain of Ammoccetes (Fig. 20) shows clearly not only 
that it is composed of two parts, an epithelial tube and a nervous 
system, but also that the nerve-masses are arranged in the same 
relative position with respect to this tube as are the nerve-masses in 
the invertebrate with respect to the cephalic stomach and cesophagus. 
This evidence is so striking, so conclusive, that it is impossible to 
resist the conclusion that the tube did not originate as part of the 
central nervous system, but was originally independent of the central 
nervous system, and has been invaded by it. 
