236 



THE DEVELOPMENT OF THE CHICK 



final termination without addition on the part of other cells. 

 The body of the neuroblast forms the nerve-cell, from which, 

 later on, secondary processes arise constituting the dendrites. 



The view that each nerve-cell with its axone process and 

 dendrites is an original cellular individual, is known as the neurone 

 theory. For the central nervous system this view is generally 

 held, but its extension to the peripheral system is opposed by 

 some on the ground that the axone in peripheral nerves is formed 

 within chains of cells, and is thus strictly speaking not an original 

 product of the neuroblast, though it may be continuous with the 

 axis cylinder process of a neuroblast. This view is discussed 

 under the peripheral nervous system. 



Each medullary neuroblast is primarily unipolar and the 



axone is the original outgrowth. 

 Soon, however, secondary proto- 

 plasmic processes arise from the 

 body of the nerve-cell and form the 

 dendrites. These appear first in 

 certain neuroblasts of the ventro- 

 lateral portion of the embryonic 

 cord, whose processes enter into the 

 ventral roots of spinal nerves (Fig. 

 140). The extent and kind of de- 

 velopment of these dendritic pro- 

 cesses of the nerve-cells varies 

 extraordinarily in different regions; 

 Figs. 139, 140, and 141 give an idea 



FIG. 140. Transverse section 

 through the spinal cord of a 

 chick on the fourth day of 



incubation; prepared by the -11 i , - ,1 



method of Golgh (After Ra- of their rapid development m the 



motor neuroblasts up to the eighth 

 day. 



The Ganglionic Neuroblasts. The 

 ganglionic neuroblasts are located, 

 as the name implies, in the series of 

 ganglia derived from the neural 

 crest. It must not be supposed, however, that all of the cells 

 of the ganglia are neuroblasts, for the ganglia contain, in all 

 probability, large numbers of cells of entirely different function. 

 (Sheath-cells, see peripheral nervous system.) It is probable 

 also that the neuroblasts of the spinal ganglia and some cranial 

 ganglia, at least, are of two original kinds, viz., the neuroblasts of 



mon y Cajal.) 



C. a., Anterior commissure. 

 D., Dendrite. d. R., Dorsal root. 

 Ep. Z., Ependymal zone. W., 

 White matter (marginal velum). 

 Nbl. 4, Neuroblast of the ventral 

 horn (motor). 



