BASIC DEVELOPMENTAL FEATURES 817 



in Chapter 10, this discrepancy in the thickness of the walls of the tube is 

 due (in the amphibia) to the inductive influence of the somite which comes 

 to lie along the lateral regions of the primitive tube. In the 9-mm. pig embryo, 

 the neural tube in transverse section begins to present three general zones 

 (fig. 353J), viz.: 



(1) an ependymal layer of columnar cells lining the lumen, 



(2) a relatively thick nucleated mantle layer occupying the middle zone 

 of the neural tube, and 



(3) a marginal layer without nuclei extending along the lateral margins 

 of the tube. 



The ependymal layer of cells lies against the internal limiting membrane 



of the tube, and consists of differentiating spongioblasts as indicated above. 

 The mantle layer contains many neuroblasts and in consequence is referred 

 to as the middle nucleated zone. It forms the future gray matter of the neural 

 tube. The outer or marginal zone in its earlier phases of development is a 

 meshwork of neuroglia and ependymal cell processes. Later, however, the 

 processes of neurons come to lie among the fibrous processes of the neurogUa 

 and ependymal cells as the nerve cell fibers extend along the spinal cord. The 

 external limiting membrane lies around the outer edge of the marginal layer, 

 and thus forms the outer boundary of the tube. In figure 353H is shown the 

 relationships of the ependymal, mantle and marginal layers of the spinal cord 

 of a 55-mm. pig embryo together with the ependymal and neuroglia cells. 

 The arrangement of the ependymal, mantle and marginal layers in the spinal 

 cord of a 22-mm. opossum embryo is shown in figure 353K. 



d. Early Histogenesis of the Peripheral Nervous System 



The formation of the cerebrospinal series of nerves which comprise the 

 peripheral nervous system involves cells located within the neural crest ma- 

 terials and also within the mantle layer (gray matter) of the neural tube. 

 One feature of the development of the spinal nerves is their basic metamerism, 

 for a pair of spinal nerves innervates the somites of each primitive segment 

 or metamere. 



The neuroblasts of each spinal nerve arise in two areas, viz.: 



( 1 ) the neural crest material which forms segmental masses along the 

 lateral sides of the neural tube, and 



(2) cells within the ventral portions of the gray matter of the tube. 



In the development of a spinal nerve bipolar neuroblasts appear within 

 the neural crest material. Each bipolar neuroblast sends a process distad 

 toward the dorso-lateral portion of the neural tube and a second process 

 lateroventrad toward the body wall tissues, or toward the viscera. Later these 

 bipolar elements become unipolar and form the dorsal root ganglion cells. 



