SYMPATHETIC TRUNKS IN CHICK 339 



the sections indicate considerable distortion. In a few segments 

 in this region in which small ventral nerve-roots are present the 

 primordia of the sjrnipathetic trunks are wanting. 



The conditions which obtain in one of the thoracic segments 

 on the side on which the spinal ganglion and dorsal nerve-root are 

 absent are illustrated in figure 1. In this segment the ventral 

 nerve-root is relatively large. There is also evidence of abundant 

 migration of cells of meduUary origin along its fibers. That 

 such migration was stiU in progress at the close of the fifth day 

 of incubation is indicated by the fact that continuous lines of 

 cells may be traced from the motor nidulus into the ventral nerve- 

 root and that cells of meduUary origin are abundant in the proxi- 

 mal portion of the nerve. Such cells are present also along the 

 fibrous communicating ramus which connects the primordium of 

 the sympathetic trunk with the spinal nerve. This primordium 

 is approximately as large as the primordium of the sympathetic 

 trunk on the opposite side in this segment where both spinal 

 ganglion and dorsal nerve-root are present. 



In order to show approximately what portion of the neural 

 tube is wanting in this segment, a camera lucida outline of the 

 entire cross section of its remnant is superimposed (figure 1, A) 

 on a camera lucida outline of the same magnification of a cross 

 section of the neural tube a few segments farther cephalad where 

 it was not affected by the operation. 



Number 2. In this embryo the operation involved the ner- 

 vous system in the lumbar and sacral regions. In the anterior 

 portion of the region involved the neural tube is ahiiost com- 

 pletely absent. In a few segments only a few small masses of 

 nervous tissue remain along the dorsal aspect of the notochord. 

 From these masses fibers which, doubtless, represent the ventral 

 roots of the spinal nerves grow out on one or both sides in each 

 segment. Continuing caudad a larger portion of the central 

 nervous system is left intact. Transverse sections show a small 

 distorted neural tube which contains a small but complete nem-al 

 canal. In a few segments small groups of nervous elements lie 

 outside the neural tube. These may or may not be cells of neural 

 crest origin. Ventral nerve-roots as well as fibers which emerge 



