Jones, Development of the Syjnpathetic. 



123 



fifth, sixth, and seventh nerves and an incipient one on the 

 eighth (Fig. 9, R.). The relations of the other nerves are the 

 same as in the preceding stage, except that the cord is better 

 defined now, and, in section, has the appearance of lying 

 against the nerve, there being fewer cells scattered among the 

 nerve fibers. (Fig. 4, Sy. shows a little later stage.) At this 

 stage, a membrane is found in many places, entirely surrounding 

 the cord and making it perfectly distinct. We can distinguish 

 now clearly for the first time two regions where the develop- 



Fig. ^. — Transverse section through an 18.5 mm. toad, just posterior to the 

 third spinal nerve (see Fig. 10), showing the sympathetic ridge, Ri., and the sym- 

 pathetic cord, Sy., well differentiated along the free border of the ridge. Refer- 

 ence marks, same as in preceding figures. X 246. Camera lucida. Reichert, 

 oc. 2, obj. 7 a. 



ment of the sympathetic as a whole has proceeded father than 

 elsewhere. These places are in the region of the brachial and 

 the lumbar plexuses. Also, the sympathetic is usually better 

 developed in the immediate region of the nerves than it is be- 

 tween them (Fig. 9). 



Eigliteen Millimeter Stage. — Between the vagus ganglion 

 and the second nerve, the sympathetic is easily traced as a large, 

 distinct, and continuous cord (Fig. 10, Sy.). Posterior to the 

 second spinal nerve, the ridge (Fig's. 5 and 6, Ri.^ now is well 



