20 Dr. A. M. Paterson. The Development of the [May l r 



ganglia or the connecting branches with the spinal nerves. The cord 

 is first seen in transverse and sagittal sections of mouse and rat 

 embryos of about 8 days. It arises on either side as a solid, 

 uniform, unsegmented rod of fusiform cells, imbedded in the meso- 

 blast surrounding the aorta, and lies in the interval between the latter 

 and the adjacent veins. Slightly thicker anteriorly than posteriorly, 

 it ends abruptly in front at the level of the first vertebral segment ; 

 behind it becomes indistinct posteriorly to the suprarenal body, to 

 which it sends a considerable cellular bundle, and, tapering off, dis- 

 appears at the level of the hind limbs. This cellular column is formed 

 by the differentiation of the mesoblastic cells in situ ; it is not con- 

 nected with the spinal nerves, and it is unsegmented. 



The next step consists in the junction of the spinal nerves on either 

 side with these columns of cells. This is effected by the gradual 

 growth of the inferior primary division of the nerve and its final 

 division at the junction of the body wall (somato-pleure) and 

 splanchnic area (splanchno-pleure) into somatic and splanchnic 

 branches (rat, mouse, 8-9 days). The former passes on to be distri- 

 buted in the body wall. The latter can be followed in succeeding 

 stages in a ventral and mesial direction, until at last it meets and 

 joins the cellular sympathetic cord (mouse, 11 days, rat, 12 days). 



The origin of this splanchnic branch is from both roots of the 

 spinal nerve, of which the ventral root contributes the greater number 

 of fibres. At its peripheral end it terminates in one of two ways. 

 At the anterior part of the thorax the fibres seem to end entirely in 

 the sympathetic cord ; that is, they have not been traced beyond it. 

 In the posterior thoracic and in the lumbar regions the splanchnic 

 branch, on reaching the sympathetic cord, divides into two parts, of 

 which one joins the cord, the other passes over it. In both cases the 

 fibres which join the cord are directly connected with the component 

 cells. 



In certain regions no such connexions can be made out. Behind 

 the kidney and the bifurcation of the aorta (i.e.. behind the loins) the 

 splanchnic branches cease. In front of the fore limbs (i.e., in the 

 neck) the splanchnic branches do not join the sympathetic system. 

 In comparatively advanced embryos, distinct nerves, morphologically 

 similar to the splanchnics, course inwards round the vertebral artery 

 to the tissues surrounding the growing vertebrae, but at the same 

 time occupying a position dorsal to the sympathetic cord, and alto- 

 gether unconnected with it. ' 



These splanchnic branches correspond to the white rami communi- 

 cantes. 



The formation of the ganglia on the main sympathetic cord occurs 

 subsequently, and is subordinate to the connexions with it of the 

 splanchnic branches of the spinal nerves. Up to the time of the for- 



