8 4 4 



DEVELOPMENT. 



[CH. LVIII. 



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

 epiblast appears and is called the neural crest. Special enlarge- 

 ments of this appear opposite the middle of each pair of proto- 

 vertebrae ; 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 contributes to the formation of 

 the posterior white column, and ultimately arborises around the 

 cells of the grey matter at a higher level. The other gi-ows to the 



Fig. 663. A, Bipolar cell from spinal gang-lion of a 4 weeks embryo (after His). , 

 nucleus ; the arrows indicate the direction in which the nerve processes grow, one to 

 the spinal cord, the other to the periphery, n, a cell from a spinal pang-lion of the 

 adult ; the two processes have coalesced to form a T-shaped junction. (Diagrammatic.) 



periphery. The two processes become blended in the first part 

 of their course and so the T-shaped junction is formed (fig. 663). 



The Brain. The histological details of the formation of the 

 epithelium of the ventricles from spongioblasts, of neuroglia from 

 the myelospongium, 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 vesicle the two primary optic vesicles are budded oft' 

 laterally : their further history will be traced in the next section. 



