734 



NEUROLOGY 



I 



or young nerve cells (Fig. 641). The spongioblasts are at first connected to one 

 another by filaments of the syncytium : in these, fibrils are developed, so that as the 

 neuroglial cells become defined they exhibit their characteristic mature appearance 

 with multiple processes proceeding from each cell. The germinal cells are large, 



Germinal cell 



Neuroblast 



Nuclei of spongioblasts 



Syncytium 



Fia. 641. Transverse section of the medulla spinalis of a human embryo at the beginning of the fourth week. 

 The left edge of the figure corresponds to the lining of the central canal. (His.) 



round or oval, and first make their appearance between the ependymal cells on 

 the sides of the central canal. They increase rapidly in number, so that by the 

 fourth week they form an almost continuous layer on each side of the tube. No 

 germinal cells are found in the roof- or floor-plates; the roof-plate retains, in certain 



Hoof-plate 



Alar lamina 



Oval bundle 



Posterior 



nerve-root 

 Central canal 



Ependymal 



layer 

 Lateral 



funiculus 

 Basal lamina 



Floor-plate 



*/ Anterior 

 nerve-root 



Anterior funiculus 



Fasciculus gracilis , Post erior 

 Fasciculus cuneatut) f 



Post, nerve-root 



Lateral 

 funiculus 



Central canal 



Anterior 

 column 



Anterior funiculus 



FIGS. 6<12, 643. Transverse sections through the medullse spinales of human embryos. (His.) 

 Fig. 642, aged about four and a half weeks. Fig. 643, aged about three months. 



regions of the brain, its epithelial character; elsew r here, its cells become spongio- 

 blasts. By subdivision the germinal cells give rise to the neuroblasts or young 

 nerve cells, which migrate outward from the sides of the central canal into the 

 mantle layer and neural crest, and at the same time become pear-shaped; the 



