CH. LIX.] FOKMATION OF SPINAL CORD AND BRAIN 861 



the anterior roots which we have already considered. The posterior 

 roots are formed in the following way : 



Along the dorsal aspect of the primitive cord a crest of epiblast 

 appears, and is called the neural crest. Special enlargements of this 

 appear opposite the middle of each pair of protovertebrae ; these grow 

 downwards on each side, and their attachment to the cord is then 

 entirely lost. These little islands of epiblast contain numerous 

 neuroblasts ; each island forms a spinal ganglion, and the neuroblasts 

 within it become the cells of that ganglion. Two processes grow 

 from each cell ; one directed towards the spinal cord, where it con- 



B. 



Fio. 659. A, Bipolar cell from spinal ganglion of a 4 weeks embryo (after His), n, Nucleus ; the 

 arrows indicate the direction in which the nerve processes grow, one to the spinal cord, the other to 

 the periphery. B, a cell from a spinal ganglion of the adult ; the two processes have coalesced to 

 form a T-shaped junction. (Diagrammatic.) 



tributes to the formation of the posterior white column, and 

 ultimately arborises around the cells of the grey matter at a higher 

 level. The other grows to the periphery. The two processes become 

 blended in the first part of their course, and so the T-shaped junction 

 is formed (fig. 659). Small portions segmented off from the spinal 

 ganglia form the sympathetic ganglia. 



The Brain. The histological details of the formation of the 

 epithelium of the ventricles from spongioblasts, of neuroglia from 

 the myelosporigium, of nerve-cells from neuroblasts, and of the 

 nerve-fibres of the white matter and of the nerves as the out- 

 growths from the neuroblasts, are all essentially the same, as already 

 described in connection with the spinal cord. But the grosser 

 anatomical details differ. 



The front portion of the medullary canal is almost from the first 

 widened out and divided into three vesicles. From the anterior 



