164 
PROFESSOR A. M. PATERSON ON THE DEVELOPMENT OF 
cord, its connexion with the spinal nerves, its terminations, cephalic and caudal, and 
its relations to the suprarenal boflies, and collateral gangdia. In the succeeding 
account the order followed will be that of development, as far as the subject under 
consideration allows, and as far as possible drawings from the actual specimens will be 
utilised to demonstrate the facts adduced. 
1. Formation of the Main Sympathetic Cord: Early Stages. 
The examination of very young Mammalian embryos leads only to negative conclu 
sions regarding the origin of the sympathetic cord. 
In Rat embryos about 5-6 days old, cut in utero, hardened in spirit and stained 
with aniline blue-black (Bevan-Lewis), the spinal cord is incompletely closed, the brain 
consists of its three primary vesicles, and the optic vesicles are in process of formation. 
In transverse sections through the thoracic region (Plate 22, tig. 1), the notochord {No.) 
is large and distinct, is surrounded by a sheath, and is in close contact with the 
spinal cord {Sg).C.). Below is the alimentary canal {Al.). in the ventral wall of which 
the first steps in the formation of the pulmonary passages are to be seen. On each side 
are the separate aortse (Ao.) and muscle plates {M.P.), the inner layers of which 
are undergoing proliferation. The dorsal ganglia {D.R.) are beginning to form, as 
indistinct buds of fusiform cells (stained like those of the spinal cord more deeply than 
the surrounding mesoblast), springing from a hardly perceptible neural ridge on the 
summit of the spinal cord. There is no sign, as yet, of the \'entral roots of the spinal 
nerves, or of the sympathetic cord. Between the muscle plate and aorta laterally, 
and the spinal cord, notochord, and alimentary canal in the middle line, is a clear 
space containing only some scattered embryonic cells with branching processes. 
In Rabbit embryos aged about 7 days (axial length 5 mm.), haA’dened in picric acid, 
and stained with borax-carmine, the dorsal aortse were fused into a single trunk, having 
a cardinal vein on each side; the lungs had begun to form, and the limljs were repre¬ 
sented as rudimentary rounded buds. The spinal cord was almost entirely cellular, a 
very narrow belt of fibres being found only in the neighbourhood of the dorsal and 
ventral roots. The development of the spinal nerves was further advsinced. In trans¬ 
verse sections through the thoracic region (Plate 22, fig. 2), the dorsal ganglion (D.G.) 
is distinctly seen : fusiform in shape, it is connected to the spinal cord (Sp.C.) by a 
bundle of fibres, among which are cells possessing large nuclei and fine tapering pro¬ 
cesses at either end. At its distal end, a bundle of fibres can be traced to join the ventral 
root. The latter is also distinct: it leaves the spinal cord as a number of separate 
fine fibres which combine with the dorsal root to form the nerve {S 2 ).n.). This can 
be traced outwards almost as far as the cardinal vein (C.V.), and to the level of the 
somato-splanchnic angle (a); or in the regions where the limbs are sprouting as far 
as the I'oots of the limbs, where in certain sections the fibres can be seen to separate 
