SUPRARENAL GLANDS 



409 



here and there in the medulla but only rarely in the cortex. A part of the 

 nerves terminate in the walls of the vessels. 



CENTRAL NERVOUS SYSTEM. 



SPINAL CORD. 



Development and General Features. The formation of the medullary 

 tube, which gives rise to the spinal cord and brain, has already been de- 

 scribed (cf. Fig. 125, p. 133); in the following section, the differentia- 

 tion which takes place in its wall will be considered, together with the 

 general features of the spinal cord in the adult. 



Very early in development, the cells of the medullary tube form a 

 syncytium. Those nuclei of the syncytium which border upon the lumen 



FIG. 423. DIAGRAMS SHOWING THB DIFFERENTIATION OF THE CELLS IN THE WALL OF THE MEDULLARY 



TUBE. (Schaper.) 



The germinal cells are stippled, and the indifferent cells are empty circles. Circles with dots represent 

 neuroglia cells, and the black cells are neuroblasts. Circles containing an z are germinal cells in mitosis. 



of the tube, or central canal, divide repeatedly by mitosis, and many of 

 them are forced outward laterally, so that the sides of the tube become 

 greatly thickened. In the floor and roof of the tube a corresponding thick- 

 ening fails to take place, as shown in Fig. 423. 



The lateral walls of the tube very early become divisible into three 

 layers (Fig. 423). The inner layer consists of germinal or proliferating 

 cells and is wide only in the embryo. In the adult it becomes reduced 

 to a single layer of inactive cells, which surround the central canal like 

 a simple epithelium and constitute the ependyma (Gr., eTrwou/xa, a cloak). 

 The middle layer is composed of cells derived from the germinal layer, and 

 in the adult it constitutes the gray substance of the cord. Its cells early 

 differentiate into two types the supporting cells, or neuroglia, and the 



