SYMPATHETIC SYSTEM IN TURTLES 307 



In a recent paper on the development of the sympathetic 

 nervous system in certain fishes,"* I have shown that neither of these 

 theories are adequate to account for the phenomena observed. 

 Furthermore, I have presented evidence in support of the view 

 that the peripheral migration of cells from the cerebro-spinal ner- 

 vous system into the sympathetic anlagen is probably determined 

 by the influence of substances, hormones, which are produced by 

 the cells in the richly nourished regions which are to become their 

 ultimate destination. 



As already indicated, before the spinal nerves may be traced 

 peripherally in embryos of the turtle cells become separated from 

 the distal ends of the spinal ganglia and migrate diagonally through 

 the mesenchyme toward the lateral surfaces of the aorta. Fibers 

 are not present along these paths nor are the cells closely aggre- 

 gated . There seems to be no purely mechanical means, therefore, by 

 which these cells could be carried forward or by which they could 

 be guided in their course. During the later stages of development, 

 as has also been shown, cells deviate from the course of the spinal 

 nerves before reaching the origin of the communicating rami and 

 advance by a more direct course toward the anlagen of the sym- 

 pathetic trunks . In this case the cells are usually more or less closely 

 aggregated, thus forming distinct cellular tracts which advance 

 mesially from the spinal nerves. Mitotic figures occur along these 

 tracts, but they are not sufficiently numerous to account either 

 for the rapid increase in the number of cells present or for their 

 advancement toward the sympathetic anlagen. We must con- 

 clude, therefore, that cells are actually displaced from the spinal 

 nerves toward the sympathetic anlagen along these cellular tracts. 

 In these later stages, as in the stages in which the spinal nerves can 

 not yet be traced peripherally, there is apparently rtothing in the 

 structure of the mesenchyme which might determine the course 

 of the peripherally advancing nervous elements. 



The phenomena above described seem to support the view that 

 the peripherally advancing nervous elements are attracted toward 

 the regions in which the sympathetic anlagen arise by the influ- 



^ Jour. Comp. Neur. Psych., vol. 21. 



