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 oesophagus ; 

 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 



