38 THE ORIGIN OF VERTEBRATES 
For the first time a logical, straightforward explanation is thus 
given of the peculiarities of the tube of the central nervous system, 
with its extraordinary termination in the anus in the embryo, its 
smallness in the spinal cord, its largeness in the brain region, and its 
offshoot to the ventral side of the brain as the infundibular channel. 
It is so clear that, if the infundibular tube be looked on as the old 
cesophagus, then its lining epithelium is the lining of that cesophagus ; 
and the fact that this lining epithelium is continuous with that of 
the third ventricle, and so with the lining of the whole nerve-tube, 
must be taken into account and not entirely ignored as has hitherto 
been the case. If, then, we look at the central nervous system of 
the vertebrate in the light of the central nervous system of the 
arthropod without turning the animal over, we are led immediately 
to the conclusion that what has hitherto been called the vertebrate 
nervous system is in reality composed of two parts, viz. a nervous 
part comparable in all respects with that of the arthropod ancestor, 
which has grown over and included into itself, to a greater or less 
extent, a tubular part comparable in all respects with the alimentary 
canal of the aforesaid ancestor. If this conclusion is correct, it is 
entirely wrong to speak of the vertebrate central nervous system as 
being tubular, for the tube does not belong to the nervous system, 
but was originally a simple epithelial tube, such as characterizes the 
cesophagus, cephalic stomach, and straight intestine of the arthropod. 
Here, then, is the crux of the position—either the so-called 
nervous tube of the vertebrate is composed of two separate factors, 
consisting of a true non-tubular nervous system and a non-nervous 
epithelial tube, these two elements having become closely connected 
together; or it is composed of one factor, an epithelial tube which 
constitutes the nervous system, its elements being all nervous 
elements. 
If this latter hypothesis be accepted, then it is necessary to 
explain why parts of that tube, such as the roof of the fourth 
ventricle, the choroid plexuses of the various ventricles, which are 
parts of the original roof inserted into the ventricles, are not com- 
posed of nervous material, but form simple single-layered epithelial 
sheets, which by no possibility can be included among functional 
nervous structures. The upholders of this hypothesis’ can only 
explain the nature of these thin epithelial parts of the nervous tube 
in one of two ways; either the tube was originally formed of nervous 
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