ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 361 



cated the problem that it is difficult to decide which of the embiyonic 

 cells form supporting structure and which nervous material. Both 

 ontogenetical and phylogeuetical evidence appears to lead to the con- 

 clusion that the central nervous system of the higher Veitebrates has 

 been formed by the spreading and increase of nervous material over the 

 walls of an original non-nervous tube, the cellular elements of which 

 tube, whatever its original function, have been utilized as supporting 

 structures for the nervous elements in those parts where the latter have 

 invaded its walls ; while in other parts, where no such invasion has 

 taken place, the walls of the tube have retained their simple cellular 

 structure, or have undergone gelatinous degeneration. 



If a comjjarison be made of the brain of Petromyzon and that of 

 Mammals we are led to the view that the nervous material of the Verte- 

 brate central nervous system is situated in definite places outside but in 

 close contact with the walls of a pre-existing non-nervous tube, and that 

 the elements of this non-nervous tube, which is formed by the folding 

 over of the medullary plate, become utilized as the supporting tissue or 

 myelospongium, wherever the nervous matter comes into contact with it. 



With regard to the embryological evidence, the difficulty lies in decid- 

 ing which of the elements of the original embryonic tube will form nervous 

 material, and which will form supporting structure ; though there has 

 been much discussion on this point. Dr. Gaskell does not think that we 

 can yet go much further than the observations of His — (1) all the 

 cells of the embryonic tube do not foi'm nervous material; (2) all the 

 motor nerve-fibres arise as prolongations of the motor nerve-cells ; 

 and (3) the motor nerve-cells, as soon as they can be recognized, are 

 always situated in a perfectly definite place in the embryonic tube, viz. 

 in the outer and not in the inner part. 



As a possible explanation of the ancestral history of the spinal cord, 

 it is suggested that it was originally composed of a bilateral chain of 

 ganglia, situated ventrally to a non-nervous tube, the parts of each 

 chain being connected together by commissures also situated ventrally 

 to this tube. By the increase and spreading round of the nerve-cells 

 and nerve-fibres to the dorsal side, the original tube was so invaded with 

 nervous elements that it lost its original character and became the 

 supporting structure of the spinal cord ; as most marked indications of 

 its original character are the epithelial lining of the central canal and 

 the peculiar structure of the substantia gelatinosa centralis. 



This definition does not, however, apply to the more anterior portion 

 of the central nervous system ; in it the ventral chain of ganglia, instead 

 of spreading round to the dorsal side of the tube, is connected by means 

 of strong encircling commissures, which form a commissural collar 

 around the tube, with a series of ganglia lying on the dorsal side of the 

 tube, whose function is of a higher character than that of the ventral 

 chain, and which give rise to no outgoing nerves, except those of such 

 special senses as sight and smell. 



Clearly this description would apply as well to an invertebrate 

 central nervous system, and, if it be true, it follows that the tube of 

 supporting tissue around and within which the nervous system is formed, 

 with its extraordinary continuation by the neurenteric canal into the 

 present alimentary canal, was originally the whole or part of the 

 alimentary canal of the invertebrate from which the vertebrate ancestor 

 arose. Further, this tube must have had an anterior as well as a 



