136 Journal of Comparative Neurology. 



Passing over to the vertebrates, we start from a stage of de- 

 velopment of the chick such as represented in the figures 10-13 

 in Dejerine's anatomy taken from Duval's atlas. The dorsal 

 lamina of the embryo shows a longitudinal groove which tends 

 to close itself In Dejerine's Fig. 12 we see the neural tube 

 almost closed. Along the dorsal suture of the tube special 

 clusters of cells are noticeable on either side which later are 

 known as 'sensory' ganglia. His called the formation 'neural 

 ridge' and we thus start with the 'neural tube' and the 'neural 

 ridges' on either side of the tube. Comparative neurology 

 shows that the elements of the neural ridge take the place, as 

 we have seen, of specialized epithelia such as are found in the 

 rain-worm's skin. These special sensory cells of the epidermis 

 send a process into one of the ganglia of the ganglionic chain. 

 (See Fig. 2.) In other worms the 'sensory' nerve-cell has its 

 cell-body beneatJi the epidermis, one process terminating in the 

 skin and the other in a ganglion of the ganglion-chain. This is 

 on the whole the type of the greatest number of afferent nerve- 

 elements of the vertebrates, the only exceptions being the ol- 

 factory and the optic apparatus, of which the former follows the 

 type of the afferent elements in Lumbricus, the cell-body of the 

 olfactory nerve fibre being among the epithelial cells of the 

 Schneiderian membrane. Only few afferent elements seem to 

 have their cell-body in the wall of the neural tube, as for in- 

 stance the mid-brain root of the fifth nerve. The Amphioxus 

 stands quite alone in having no neural ridges ; all the afferent 

 elements (spinal ganglion elements) have their cells in the wall 

 of the neural tube. 



The vertebrate body is to a certain degree segmented. 

 This is clearly shown in the general aspect of the nervous sys- 

 tem. We can divide the body by transverse sections into 

 laminae which show a certain harmony of architecture within 

 the region of the vertebrae, and, for our purposes, especially by 

 the peripheral nerve-roots which come forth through the inter- 

 vertebral foramina. The constant repetition of the type : 

 vertebra-ncrvc-root-vertebra with the corresponding piece of the 

 neural tube * belonging to' the nerve-root constitutes the justifi- 



