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. 
‘ach 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- 
Fic. 140.— Transverse section cesses of the nerve-cells varies 
through the spinal cord of a extraordinarily in different regions; 
chick om the fourti) day ‘of “Wigs. 139,140) and 141 give amaded 
incubation; prepared by the Lana Ae ie ~ fama 
‘1s heir rar velo nt 4 
method of Golgi. (After Ra- © SE ee ee oy = 
mon y Cajal.) motor neuroblasts up to the eighth 
©. a., Anterior commissure. day. 
D., Dendrite. d. R., Dorsal root. The Ganglionic Neuroblasts. The 
ip. Z., Ependymal zone. W., ee 
White matter (marginal velum). ganglionic neuroblasts are located, 
Nbl. 4, Neuroblast of the ventral 
as the name implies, in the series of 
horn (motor). 
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 
