THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
167 
nerves. It is slightly thicker in front than behind, but is of almost uniform width 
throughout. It presents no evidence whatever of segmentation. 
I have not the slightest hesitation in asserting that the cellular column above 
described is the precursor of the main sympathetic chain. It occupies the position of 
that chain; and there is no other structure to be found in the vicinity, in the form 
of isolated ganglia or otherwise, wliich might lead one to doubt its significance. It is 
mesodermal in origin; it is formed in situ; and it is unsegmented and unconnected 
with the spinal nerves. The next step in the investigation is to discover how this 
connexion takes place. 
2. Connexion of the Sympathetic Cord and Spinal Nerves: Formation of the Kami 
communicantes. 
In Kat embryos, aged between 8 and 9 days, axial length 8’5 mm., hardened in 
spirit, and stained with aniline blue-black, the above-mentioned cellidar cord {Sy., 
Plate 23, fig. 5) is slightly larger than before. In transverse sections it has the 
same close connexion with the aortic wall (Ao.) ; and in certain places can be traced 
into continuity with small bundles of similar cells, which form groups lying against 
the sides of the vessel, and tapering off in a ventral direction. These cells form the 
first sign of peripheral distribution from the cord. At this date the cord itself 
presents no sign of constriction or segmentation, and has the same antero-posterior 
limits as before. 
It is in the spinal nerve that the most significant change is now seen. In j^revious 
sections the inferior primary division was traced as far as the cardinal vein, above the 
somato-splanchnic angle. It is now seen (Sp.N) to divide at that spot (a) into two 
unequal parts, a larger external (So.) and a smaller internal branch both of 
which are very short and end abruptly in a fringe of nerve fibres lying between and 
against scattered mesoblastic cells. These two short branches form the first appear¬ 
ance of the somatic and splanchnic branches of the nerve. The former is directed 
towards the body wall, the latter inwards, behind the cardinal vein (C.V.) towards 
the median line. 
In Mouse embryos of about 9 days, hardened in picric acid, and stained with borax- 
carmine, development has f)ei’ceptibly advanced. Transverse sections (Plate 23, 
fig. 6) show the spinal cord (*^9.C.) with its cells arranged in two groups; a broad 
band of deeply stained cells surrounds the neural canal, while more externally a 
mass of paler and more scattered cells is found, the precursor of the proper gray 
matter of the cord. Outside the cells the belt of peripheral fibres has become 
distinctly thicker. The cellular cord (Sy.) presents the same characters as before ; 
it is thick anteriorly, gradually tapering oft' posteriorly ; and it possesses the same 
bundles of cells in connexion with it, passing ventrally, and embracing the sides of 
the aorta. In the spinal nerve the condition of development is advanced. The 
superior primary division is plainly seen; the somatic branch (So.) of the inferior 
