792 
ROSS GRANVILLE HARRISON 
EARLY DEVELOPMENT OF NERVE FIBERS IN THE NORMAL 
EMBRYO 
Conditions obtaining in the central nervous system antecedent to the 
differentiation of fibers 
In the walls of the medullary groove and the medullary tube 
just after it has been completely folded off from the epidermis, one 
can distinguish in sections a number of irregular layers of cells, 
mostl}^ oval in shape, with long axis placed radially with respect 
to the tube as a whole. Sometimes these cells are seen to be 
bound by a membrane, but usually they are indistinctly defined 
except where they are deeply pigmented, in which case the pig- 
ment granules are thickest around the periphery of the cells. At 
this period the individual cells do not extend through the whole 
thickness of the tube from the central canal to the external 
limiting membrane. 
In slightly later stages, i.e., when the tail bud is barely distin- 
guishable, the epithelial cells begin to stretch out radially and 
then many of the individual cells are seen to extend from the inner 
to the outer wall of the tube. The boundaries remain indistinct, 
unless, as before, the cells are marked off from their neighbors 
by pigmentation. After the elongation of the epithelial cells 
■constituting the walls of the medullary tube has taken place, it 
is seen that certain cells, less elongated in form, and containing 
a round nucleus, remain in the outer zone of the wall of the tube. 
These are the first of the neuroblasts of His, the cells destined to 
give rise to the nerve fibers. 
There are as yet no peripheral nerves, nor are there any nerve 
fibers visibly differentiated within the walls of the medullary 
tube. The cranial ganglia are marked off and occupy approx- 
imately their definitive position, and in the anterior part of the 
trunk region the ganglion crest is beginning to break up, its cells 
extending to the dorsal border of the muscle plates. In the mid- 
dle of the trunk the crest is intact and it rests entirely upon the 
medullary cord, while near the tail bud it can scarcely be distin- 
guished at all. 
