HISTOGENESIS OF SYMPATHETIC SYSTEM 23 
majority of the cells which enter the primordia of these ganglia, 
as indicated above, advance from the neural tube via the paths 
of the ventral roots of the spinal nerves. These cells arise in 
portions of the neural tube which give rise to efferent elements. 
Doubtless, many of them give rise to sympathetic neurons. On 
the other hand, some cells become displaced from the spinal 
ganglia, which are essentially sources of afferent neurons, into 
the sympathetic primordia. Do these cells also give rise to 
sympathetic neurons? ‘The present data do not warrant a 
conclusive answer to this question. Doubtless, many of the 
cells which enter the sympathetic primordia from the spinal 
ganglia became incorporated in the neurilemma of the visceral 
afferent fibers of the spinal nerves. Possibly others give rise 
to supporting elements in the sympathetic ganglia. The ex- 
perimental data here presented are not incompatible with the 
theory that only cells of medullary origin give rise to neurons in 
the ganglia of the sympathetic trunks and the prevertebral 
plexuses. However, a definite conclusion on this point awaits 
further experimentation. 
The cardiac and the pulmonary plexuses, doubtless bear the 
same genetic relationship to the hindbrain as do the sympathetic 
trunks and the prevertebral plexuses to the spinal medulla. 
According to one school of physiologists, the sympathetic 
nervous system contains no afferent neurons. Nevertheless, 
local reflexes are known to occur in the walls of the digestive 
tube in the absence of extrinsic nerves. This fact suggests the 
presence of afferent neurons in the enteric plexuses. TF urther- 
more, the morphological relationships of the neurons in these 
plexuses, as described by the present writer (13) and by Miil- 
ler (20), suggest that the enteric plexuses constitute a local 
reflex mechanism involving both afferent and efferent neurons. 
If this interpretation be correct, it might logically be assumed 
that the afferent neurons in these plexuses arise from cells which 
migrate from the vagus ganglia, while the efferent neurons arise 
from cells which advance from the walls of the hindbrain. 
The homology of the cells of nervous origin which advance 
into the primordia of the sympathetic nervous system with the 
