Jones, Development of the Sympathetic. 1 19 



(Fig's. I and 7, Ma.^ with which the sympathetic ridge is close- 

 ly connected, is rather small and ill-defined anteriorly. In the 

 region of the fourth nerve, however, it becomes more distinct 

 (Fig. 7), and graduall}' increases in size from here back to the 

 anterior end of the kidney, where it lies in contact externally 

 with this organ instead of the Wolffian duct. Fig. i, Ma. 

 shows this mass of cells. Between the aorta [Ao. /.) and the 

 Wolffian duct {W. D.), on the left side, it represents the con- 

 dition anterior to the kidney; between the aorta {Ao. i.) and 

 the kidney {K.), on the right side, it shows the condition in the 

 region of the kidney. 



Midway between the third and fourth nerves, a few cells ly- 

 ing close against the side of the aorta are seen to form a rather 

 poorly defined ridge (Fig. i, Rt.), which can be traced back 

 without interruption to the region between the ninth and tenth 

 nerves (Fig. 7, Ri.). It has not yet made its appearance in the 

 region of the third nerve, but traces of it are seen immediately 

 behind the second. The cells of this ridge-like elevation consti- 

 tute the earliest structure to ivJiich I have traced the sympathetic sys- 

 tem in the region posterior to the second spinal nerve. The source 

 of the cells constituting the sympathetic back of the second spin- 

 al nerve is still uncertain. I have no definite clue as to their 

 origin, outside of the fact that these cells mingle freely with the 

 nerve fibers as far back as the ninth. This close connection be- 

 tween the nerves and the sympathetic ridge suggests, at any 

 rate, that the cells are of epiblastic origin. 



At this stage, then, the following points are especially to 

 be noted: (i) Between the vagus ganglion and the second 

 nerve, scattered cells are becoming aggregated to form the sym- 

 pathetic cord (Fig. 8, Sy.^. (2) In the region posterior to the 

 second nerve, the ridge-like structure from which the sympathe- 

 tic arises (Fig's, i and 7, Ri.^ extends from the second nerve back 

 to the region between the ninth and the tenth, being interrupted 

 in the region of the third. (3) The cells of the forming sympa- 

 thetic mingle freely with the fibers of the first nine spinal nerves. 



Twelve Millimeter Stage. — In this embryo, between the 

 vagus ganglion and the second spinal nerve, we find the sympa- 



