THE NEUROCCELIA AND THE NEUROGLIA. 93 



g. In the forebrain, and in the midbrain regions, the medullary cords, in both 

 vertebrates and arachnids, remain practically horizontal. As they are very 

 broad (the diencephalon alone showing a marked lateral compression), and as 

 the overgrowth is almost entirely from the sides, the resulting cavities are broad 

 and shallow. The roof is epithelial in character, and all the nervous material, 

 except the optic ganglia and the stomoda^al commissures, forms the floor of the 

 chamber. 



10. In the region of the cord, however, the infolding combines another factor, 

 especially prominent in the vertebrates, in that, as the median groove deepens, the 

 two cords close like the covers of a book, bringing their outer, or neurogenic sur- 

 faces, face to face, converting the neural groove into a deep lying canalis centralis, 

 and reducing the marginal overgrowths to the narrow strip of epithelium that 

 roofs over the posterior fissure. (Figs. 134, 137 and 231.) 



VI. THE NEUROGLIA. 



The neuroglia arises from the epithelial lining of the middle cord groove or 

 canal. Referring to the early stages of Acilius (Fig. 221), it will be seen that the 

 medulla is invaded by numerous small, dark nuclei, that spread out laterally from 

 the middle cord, forming a uniform envelope about the medulla, the so-called inner 

 neurilemma. Later these cells multiply and invade the medulla and surround the 

 nerve cells, forming a coarse, nucleated reticulum or neuroglia. 



In the adult, this tissue is easily recognized. In sections of the brain and cord 

 of Limulus stained in haemotoxylin, Lyons blue, and acid fuchsin, it is intense red, 

 and the nerve fiber masses blue. In preparations treated by von Rath's method, 

 it is intense black, nerve fibers gray. In sections of the adult cord (Figs. 67 and 

 68), the neuroglia network may be seen springing in root-like processes from the 

 thick layer lining the central canal, as well as from the sides of the neural fissures 

 and the surface of the medulla. 



