THE CENTRAL NERVOUS SYSTEM. 



333 



round cells invading the inner zone represent the ancestors of the 



nervous elements both cells and fibres and are the germ-cells, 



while the columnar cells 



produce the neuroglia FlG - 357- 



and are the spongio- 



blasts. 



The development of 

 the nerve-cells proceeds 

 from the germ -cells, 

 which, as shown by the 

 karyokinetic figures within 

 their nuclei, undergo ac- 

 tive proliferation, the 

 resulting progeny being 

 the neuroblasts, from 

 which the nerve - cells 

 are directly derived. The 



germ-cells are confined to the zone next the brain-cavity, 

 which thus indicates the position of greatest formative energy. 



The neuroblasts at first occupy the innermost zone, next the 

 cerebral cavity, but soon migrate towards the outer boundary of the 

 wall, at the same time becoming pyriform and elongated. The 

 young nerve-cells for a long time possess but a single process, 



Section of ten-day rabbit embryo, showing closed neural 

 tube : n, neural canal ; s, area from which segmental gan- 

 glia develop ; m, mesodermic tissue ; g, gut-tube ; v t z>, 

 primitive aortae ; /, pleuro-pericardial cavity. 



FIG. 358. 



FIG. 359. 



Primary wall of neural tube composed 

 of single layer of epithelium () ; b, b, 

 germ -cells occupying inner zone. (After 



Portion of inner zone of wall of neural 

 tube in which round germ-cells (, ) 

 and partially-developed neuroblasts (nf, 

 n') lie among the surrounding spongio- 

 blasts. (After His.) 



which grows out to become a nerve-fibre, and therefore represents 

 the axis-cylinder, or nerve-process; the protoplasmic pro- 



