490 



TEXT-BOOK OF EMBRYOLOGY. 



send radial extensions into the wall of the neural tube (Figs. 421 and 422). 

 These cells develop cilia projecting into the lumen. 



A still later differentiation in the supporting elements of the tube is the ap- 

 pearance of neuroglia fibers — a product of the spongioblastic protoplasm, but 

 differing from it chemically (Fig. 423). The exact relation of these neuroglia 

 fibers to the nucleated neuroglia cells in the adult is a matter of dispute. 







Fig. 423. — Hardesty. Combination drawing from transverse sections of the spinal cord of 20 cm. 

 pig. Showing the first appearance of neuroglia fibers, a, Neuroglia cell as shown by the 

 Benda method of staining; a', similar cell by the Golgi method; b and &', non-nucleated 

 masses; d, free nuclei; e and/, differentiating neuroglia fibers; s, ''seal-ring" cells, envelop- 

 ing myelinating nerve-fibers. 



With the penetration of blood vessels into the neural tube a certain amount of 

 mesodermal tissue is brought in. How much of the supporting tissue of the 

 nervous system is derived from the mesoderm is uncertain, but it is most 

 probable that it is relatively small in amount and is confined principally to the 

 connective tissue of the walls of the blood vessels. 



Early Differentiation of the Nerve Elements. 



It has been seen that some of the actively dividing cells (germinal cells) at 

 first simply increase the ordinary epithelial elements of the tube which in turn 

 form the myelospongium, the spongioblasts and finally the ependyma and the 

 neuroglia. Other daughter cells produced by the division of the germinal cells 



