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 Ammoccetes, 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. 



