CH. LVIH.] DEVELOPMENT OF SPINAL CORD. 843 



the fissures of the cord begin to appear. The anterior or ventral 

 fissure is simply a cleft between the enlarging lateral halves of the 

 cord. The posterior fissure is formed by the closure of the dorsal 

 portion of the neural canal which meets an ingrowth of connective 

 tissue from the exterior. The characteristic cylindrical form of the 

 cord is attained by the development of the lateral columns, which 

 are formed by the processes from neuroblasts in the brain growing 

 down the sides of the cord, and these become medullated at a 



Fig. 662. Section of spinal cord of a four weeks human embryo. The posterior roots are 

 continued within the cord into a small longitudinal bundle, which in 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 myelo- 

 spongium, compose the grey matter. (His.) 



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 pasw, 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 fil-um ternrinale form the cauda equina. 



The Nerves. These grow from the spinal cord ; the origin 

 of the anterior roots we have already considered. The posterior 

 roots are formed in the following way. 



