HISTOGENESIS OF SYMPATHETIC SYSTEM 15 
tal to the visceral rami and distinguished from the cells of the 
surrounding mesenchyme by the greater size and more elongated 
form of their nuclei. The cells in this portion of the nerve are 
identical in appearance with the cells which enter the primordia 
of the sympathetic trunks. They are also intimately associated 
with the growing nerve-fibers. Obviously, they are cells which 
become incorporated in the neurilemma. 
The origin of the neurilemma of the efferent fibers of the 
spinal nerves has been studied by not a few investigators. Kupf- 
fer (94, ’95), Hoffmann (97), Harrison (01), and Neal (03) 
presented evidence, based on studies of selachian and fish em- 
bryos, which indicates that the elements which give rise to the 
neurilemma of the fibers of the ventral nerve-roots are cells of 
medullary origin which advance peripherally along these fibers. 
In his experimental studies of the development of the peripheral 
nerves, Harrison (’04) described the spinal nerves, including 
only ventral root fibers, which arose in embryos of the frog (Rana 
esculenta) in which the neural crests and the dorsal portion of 
the neural tube had been extirpated early, as composed of naked 
fibers; i.e., fibers which are unaccompanied by cellular elements. 
This finding seemed to justify the conclusion that, in the Am- 
phibia, the neurilemma of both the dorsal and the ventral roots 
of the spinal nerves is derived from cells which advance from the 
spinal ganglia. This result of Harrison’s experimental work 
has exerted great influence on subsequent investigations and on 
the more recent literature bearing on the development of the 
peripheral nervous system. Therefore, it seems worth while to 
present here certain evidence afforded by embryos of the frog 
included in the present series. The naked character of the 
fibers of the ventral roots of the spinal nerves in the absence 
of spinal ganglia and dorsal roots was confirmed by Harrison 
(06) in the American species R. sylvatica and R. palustris. 
Figure 8 illustrates microphotographically the ventral root 
of a spinal nerve in an embryo of the frog (9) in which the spinal 
ganglia and dorsal nerve-roots are absent. The neural crests 
and the dorsal portion of the neural tube were extirpated pos- 
terior to the head by cutting a strip of tissue from the dorsal 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 1 
