DEVELOPMENT OF THE SPINAL CORD. 57 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



As has been already described, the whole of the central nervous system takes origin 

 from the thickened walls of a dorsally situated axial groove, subsequently converted 

 into a canal, which rims forwards in front of the primitive streak, and the anterior 

 end of which becomes enlarged and converted by constrictions into three successive 

 vesicles, around which the several parts of the brain are formed, and which are 

 known as the primary cerebral vesicles. The remainder of the neural canal is of 

 nearly uniform diameter, and its walls become converted into the substance of the 

 spinal cord, while the cavity itself becomes eventually the central canal of the cord. 

 The walls of the neural groove are of course composed of epiblast, and it therefore 

 follows that the whole structure of the central nervous system is laid down in 

 epiblast, and consists in the main of more or less modified epiblastic elements, except 

 where mesoblastic tissues subsequently penetrate into it, conveying blood-vessels 

 into its substance. As was shown by Balfour, the same is in all probability true for 

 all the nerves of the body, cranial and spinal, which either, as with the fibres of the 

 anterior roots of the spinal nerves, grow directly out from the neural epiblast, or, as 

 with the fibres of the posterior roots, are formed and grow from masses of epiblast 

 cells, which are separated off at the junction of the neural and general epiblast to 

 form the ganglia, from which the posterior root fibres appear to take origin (His). 

 An exception must, however, be recorded for the olfactory tracts and bulbs and optic 

 tracts and nerves, which, although derived from the neural epiblast, yet have a different 

 mode of origin from all other nerves, both cranial and spinal, since they arise not as 

 solid outgrowths of that epiblast, but as hollow protrusions from the brain, which 

 only become solid at a later stage of development. We have then to consider the 

 manner in which are developed (1) the spinal cord ; (2) the several parts of the 

 brain ; and (3) the spinal and cranial nerves and their ganglia, as well as the ganglia 

 and nerves of the so-called sympathetic nervous system. 



DEVELOPMENT OP THE SPINAL CORD. 



Soon after the neural canal is closed (fig. 32, p. 31), it takes the form, along the 

 greater part of the length of what is afterwards to become spinal cord, of a cleft- 

 like cavity, with thick sides, and a relatively thin dorsal and ventral boundary (roof 

 and floor). The parietes of the canal are wholly composed of long columnar epithe- 

 lium cells, whose free borders, which are at first smooth, but later become ciliated, 

 line the cavity, and whose attached extremities rest upon a homogeneous limiting 

 membrane which early makes its appearance, bounding the embryonic cord, and 

 separating it from the surrounding structures. These cells, therefore, extend at 

 first through the whole thickness of the embryonic cord, and they have the closely- 

 set, palisade-like character, with the nuclei at different depths, such as it is usual 

 to find in long columnar epithelium. 



After a time, it is found that the ceUs (which have become always longer with 

 the increasing thickness of the wall of the neural canal) show a tendency to branch 

 and to unite with the branches of neighbouring cells. In this way a network or 

 spongework is produced, which extends throughout the greater part of the thickness 

 of the embryonic cord ; at the same time the inner parts of the cells which immedi- 

 ately line the canal retain their palisade-like arrangement, while the external or 

 attached ends often exhibit a radiating disposition, which gives a characteristic 

 radial character to the external layer of the reticular structure. The reticulum is 



