THE SYMPATHETIC KERVOUS SYSTEM IN MAMMALS. 
179 
their processes with one another and with the cells of the cord ; and, especially in the 
hinder part of the thoracic region, form considerable masses traceable along the main 
vessels. In older embryos (Plate 29, fig. 18, Pat, 22 days) they form ganglia and 
nerves, in the way already described for the main cord; and, in the region referred 
to, give rise to the Splanchnic nerves, semilunar ganglia, &c. These branches appear 
to follow in their development the same line of growth as the splanchnic branches of 
the spinal nerves ; although they are equally evident in places where the latter are 
not seen. 
(c.) The mode of development of the gray Rami commiiniccmtes I have not made 
out with perfect satisfaction. They appear to take their origin from the sympathetic 
cord as cellular outgrowths, which in some cases are traceable along the splanchnic 
branches of the spinal nerves towards their roots; but on this question I must 
withhold a definite opinion until supported by more conclusive evidence. I have not 
been able to satisfy myself about their formation in places where the splanchnic 
branches are absent, or are unconnected with the sympathetic cord. 
(cZ.) The relation of the sympathetic cord to the development of the suprarenal 
bodies is very evident in younger embryos. A considerable column of cells (Plate 27, 
fig. 14a) can be traced in sagittal sections from the ventral aspect of the main cord, 
downwards and backwards, to join a mass of mesoblastic cells situated at the anterior 
end of the embryonic kidney {K.). Entering this mass the cellular column spreads 
out, so as to constitute the central portion or medulla of the suprarenal body (Sr.C.), 
while the tissue around forms the cortex. This agrees, in general terms, with the 
mode of development of the suprarenal bodies described by previous observers (13, IG, 
17, 18). It must be borne in mind that according to this view the ivliole of the 
Mammalian suprarenal capsule is mesoblastic. The connecting stalk becomes con¬ 
verted into nerve fibres connecting the suprarenal body to the sympathetic cord. 
III. Conclusions, 
A. The Develojiment of the Sympathetic System. 
The following are the principal conclusions derived from the preceding investiga¬ 
tions :—■ 
1. The formation of the main sympathetic cord is the first event. It is developed 
as a cellular rod or column, uniform in outline, and without ganglia or constrictions. 
It is formed in, and derived from the mesoblastic tissue on either side of the embryonic 
aorta, and in front of the growing vertebral column. The cord appears after the 
formation of the roots and ganglia of the spinal nerves, and is entirely independent of 
them at first, 
2. The connexion of the spinal nerves with the main sympathetic cord occurs 
secondarily. The inferior primary division of a (typical) spinal nerve divides, on 
reaching the junction of body wall and splanchnopleure, into a somatic and a splanchnic 
2 A 2 
