816 THE NERVOUS SYSTEM 



neural tube. However, two main morphological types of cells may be 

 considered: 



( 1 ) One type of neuron possesses a short axon or axis cylinder. This type 

 of neuron lies entirely within the gray substance of the neural tube. 



(2) In a second type of neuron a long fiber or axis cylinder is developed 

 and this fiber leaves the gray substance and traverses along the white 

 substance of the cord or within the fiber tracts of the forming brain. 

 In many instances, the cell body of the second type of neuron lies 

 within the gray matter of the spinal cord, but its axis cylinder passes 

 out of the nerve tube as the efferent or motor fiber of a spinal or cranial 

 nerve (fig. 355F and I). 



b. The Development of the Supporting Tissue of the Neural Tube 



The potential connective tissue cell of the neural tube is the spongioblast. 

 Spongioblasts are of ectodermal origin and differentiate into two main types 

 of cells: (1) Ependymal cells, and (2) neuroglia cells. 



Spongioblasts together with primitive neuroblasts lie at first within the 

 ependymal zone of the neural canal particularly close to the lumen. Cilia 

 are developed on the free surface of each spongioblast lining the neurocoel. 

 From the opposite end of the cell, that is, the end facing the periphery of the 

 tube, an elongated process extends peripherad to the outer surface of the 

 neural tube. In this way a slender framework of fibers extends radially across 

 the neural tube, from the lumen to the periphery (fig. 353F-K). A spongio- 

 blast which retains a relationship with the lumen and at the same time possesses 

 a fiber extending peripherad is known as an ependymal cell. The ependymal 

 cells thus are those cells whose bodies and nuclei lie next to the lumen of 

 the developing spinal cord and brain but possess processes which radiate out- 

 ward toward the periphery of the cord (fig. 353A and F). The peripheral fiber 

 or extension may be lost in the later ependymal cell together with its cilia. 



In fishes and amphibians the supporting elements of the central nerve tube 

 retain the primitive arrangement outlined above (see Ariens-Kappers, '36, 

 p. 46). However, in reptiles, birds and mammals, the radial pattern of many 

 of the primitive spongioblasts is lost, and these spongioblasts transform into 

 neuroglia cells, losing their connection with the lumen and with the external 

 limiting membrane of the tube (fig. 3531). 



c. Early Histogenetic Zones of the Neural Tube 



The neural plate of the late gastrula is a thickened area of cells of about 

 3 to 4 cells in thickness. As the neural plate is transformed into the neural 

 tube the majority of the neural plate cells become aggregated within the lateral 

 walls of the tube. The lateral walls of the developing neural tube in conse- 

 quence are thicker than the dorsal and ventral regions. As already observed 



