DEVELOPMENT OF THE SPINAL NERVES. 619 
The dorsal root is the first to appear,—-before, during, or after the union of the 
medullary plates and the formation of the neural tube. It takes origin as a cellular bud from 
the dorsal surface of the medullary tube in one of three ways :—(1) It may arise from the 
junction of the medullary plate and surface epiblast before the closure of the medullary 
eroove. (2) It may spring from the neural crest, a ridge on the dorsal aspect of the 
medullary tube, after its closure is complete. (3) It may be simply a direct outgrowth 
from the dorsal surface of the medullary tube. Pyriform in shape, the bud enlarges and 
becomes separated from the medullary tube, and projects ventrally in the space between 
the myotome and the medullary tube. Each bud is separated by only a slight interval 
from its neighbour. 
The cells (neuroblasts) composing the bud become rapidly spindle-shaped, and by the 
middle of the fourth week give rise to two sets of processes ; (1) a central series, which 
Fria. 450.—DEVELOPMENY OF THE SPINAL NERVES. 
A, Formation of nerve roots. B, Formation of nerve trunk (N) 
D.R, Dorsal root. Al.C, Alimentary canal. D.G, Dorsal ganglion. 
V.R, Ventral root. Ao, Aorta. 5 Sy, Sympathetic cord. 
N.T, Neural tube. f Ve Cardinal vein. W.D, Wolftian duct. 
No, Notochord. M.P, Muscle plate. Co, Ceelom. 
C, Formation of nerves. D, E, Formation of subordinate EF, Formation of nerve trunks in relation 
Semisonic division branches. to the limb: dorsal and ventral 
: : : 
TAN Geral branch Late lateraleand trunks corresponding to lateral and 
; 5 c . 4achh, , > ¢ . : 7 
P, Posterior primary division. Ant, Anterior, branches. anterior trunks in D. and E. 
erow backwards and are secondarily connected with the dorso-lateral aspect of the 
medullary tube as the fibres of the dorsal root; and (2) a peripheral series, which con- 
stitute the dorsal root fibres of the spinal nerve and join the ventral root, to form the 
spinal nerve proper. The intermediate cellular mass remains as the spinal ganglion. 
The ventral root of a spinal nerve arises in quite a different way, from cells (neuro- 
blasts) in the substance of the medullary tube. In the account of the development of the 
spinal cord it has been shown how the cellular constituents of the medullary tube are con- 
verted into two classes of cells: (1) spongioblasts, which produce the matrix (neuroglia) of 
the spinal cord ; and (2) germ-cells or neuroblasts, which produce the nerve-cells of the 
gray matter of the cord. The neuroblasts give rise to the axis-cylinder processes or 
axons, which, penetrating the spongy tissue of the medullary tube and the outer limiting 
ae , 
