HISTOGENESIS OF SYMPATHETIC SYSTEM 17 
ent also in the ventral rami distal to the level at which the cells 
which enter the sympathetic primordia deviate from the course 
of the latter. Obviously, these cells are destined to give rise 
to neurilemma; consequently, the conclusion that in this species 
cells of medullary origin become incorporated in the neurilemma 
is Justified. 
THE HISTOGENESIS OF THE VAGAL SYMPATHETIC PLEXUSES 
A review of the older literature on the development of the 
sympathetic nervous system reveals no detailed studies on the 
histogenesis of the peripheral sympathetic plexuses which are 
functionally related to the vagi, viz., the pulmonary, the cardiac, 
and the enteric plexuses. The older investigators quite gener- 
ally supported the theory that the primordia of these plexuses 
arise from cells which become displaced peripherally from the 
primordia of the sympathetic trunks. 
As early as 1909, the writer presented evidence, based on a 
study of embryos of the pig, which indicates that the pulmonary, 
the cardiac, and the enteric plexuses, except in the aboral por- 
tion of the digestive tube, arise from cells of cerebrospinal ori- 
gin which advance peripherally along the paths of the vagi. 
This evidence was corroborated by the later studies of the writer 
based on embryos of types of all the classes of vertebrates above 
the Cyclostomata. It was further corroborated by the work 
of Abel (12) on embryos of the chick. More recently, Miiller 
(20) traced the cells which give rise to the enteric plexuses in 
selachian embryos both from the vagus ganglia and the sym- 
pathetic trunks. Stewart (’20), whose studies are based on 
embryos of the rat, states “‘that part if not all of the nerve cells 
found in the cardiac, gastric, tracheal, oesophageal, pulmonary, 
and upper intestinal plexuses are of vagus origin.” 
The problem of the histogenetic relationships of the vagal 
sympathetic plexuses which are functionally related to the vagi 
may be attacked experimentally in two ways. If the cerebro- 
spinal nervous system could be destroyed or extirpated through- 
out the trunk region before cells of nervous origin have advanced 
peripherally and the embryo should continue to develop without 
