860 



DEVELOPMENT 



[GIL LIX. 



columns simultaneously begin to appear on each side of the narrow 

 dorsal part of the canal. They are formed by the posterior roots 

 entering the cord. 



As the cornua of grey matter grow out from the central mass, the 

 anterior fissure and the posterior septum of the cord begin to appear. 

 The anterior or ventral fissure is simply a cleft between the enlarg- 

 ing lateral halves of the cord. The posterior septum is formed by a 

 condensation of the neuroglia in the dorsal wall of the neural canal. 



FIG. 658. Section of spinal cord of a four weeks human embryo. The posterior roots are continued 

 within the cord into a small longitudindal bundle, which is the rudiment of the posterior white 

 column. The anterior roots are formed by the convergence of the processes of the neuroblasts. 

 The latter, along with the elongated cells of the myelospongium, compose the grey matter. (His.) 



The cylindrical form of the cord is attained by the development of 

 the lateral columns, which are formed by the processes from neuro- 

 blasts in the brain growing down the sides of the cord, and these 

 become medullated at a later period. The membranes and blood- 

 vessels are formed from mesoblast. 



Up to the fourth month the cord and vertebral canal increase in 

 length pari passu, but after that, the vertebral canal grows faster, so 

 that at birth the coccygeal end of the cord is opposite the third 

 lumbar, and in the adult opposite the first lumbar vertebra. This 

 gives an obliquity to the lower nerve roots, which, together with the 

 filum terminate, form the cauda eguina. 



The Nerves. Some fibres grow from the spinal cord and form 



