NERVOUS SYSTEM 



835 



throughout their course which tends to show that the vertebrate condi- 

 tion is not primitive. 



THE SPINAL CORD. 



After the neural tube has formed by a joining in the dorsal midline 

 of the two folds of the neural plate, the cells on each side of the neural 

 tube proliferate very rapidly while those of the roof and floor do not. 

 Thi^s. causes an outgrowth of the two sides so that a fissure (Fig. 470) or 



Fig. 470. 

 Cross section of spinal cord. A, "spider" cells, B, "mossy" cells. 



groove is formed on the ventral surface running the whole length of the 

 cord. In fact, the cells on the side have already begun to proliferate be- 

 fore the closing of the tube. There is an ingrowth of connective tissue 

 and blood vessels on the dorsal midline which forms a posterior or dorsal 

 septum dividing the dorsal part of the cord into halves. The entire lining 

 of the central canal, composed of epithelial cells, is known as ependyma 

 and while no definite nervous cells can be seen, it is sensory, and remains 

 sensory throughout its entire career. 



The remaining cells on each side develop into two kinds of cells, one 

 called neuroglia or simply "glia" cells, which are used to support the 

 true nerve cells ; the others form neuroblasts which develop true nervous 

 tissue. This latter type of cell must develop a fibre in order to connect 

 with other cells and with other portions of the body. These are formed 



