SYMPATHETIC SYSTEM IN TURTLES 283 



these paths, the peripheral advancement of the cells cannot be 

 accounted for by the pressure due to mitotic division. These 

 elements are too loosely aggregated to permit of being pushed 

 forward by the pressure which might be exerted by the mere 

 crowding due to the multiplication of cells. Neither is there any 

 apparent line of weakness in the mesenchyme which might deter- 

 mine the path of these migrant nervous elements. The course 

 taken by them is approximately the most direct course from the 

 distal ends of the spinal ganglia into the regions in which the an- 

 lagen of the sympathetic trunks arise (fig. 1, M. C). 



At the close of the ninth day of incubation the anlagen of the 

 sympathetic trunks are present from the cervical to the sacral 

 region. The cell-aggregates constituting the anlagen of the sym- 

 pathetic ganglia appear in tranverse sections as oval or elongated 

 cell-masses lying along the lateral surfaces of the aorta and along 

 the dorsal surfaces of the carotid arteries. These cell-aggregates 

 are best developed in the thoracic and in the dorsal region, i. e., 

 in the regions in which the sympathetic anlagen first arise. The 

 spinal ganglia are now well differentiated and the fibers of the 

 spinal nerves may be traced peripherally for some distance 

 beyond the level of the aorta. The anlagen of the ganglia of 

 the sympathetic trunks are connected with their respective nerves 

 by cellular tracts. In some sections fibers appear in the proximal 

 parts of these cellular tracts, constituting the earliest fibers of 

 the communicating rami. The cellular tracts extending from 

 the distal ends of the spinal ganglia into the anlagen of the sym- 

 pathetic trunks, which were so conspicuous in the previous stage, 

 no longer appear. The cells which become separated from the 

 cerebro-spinal nervous system in the region of the trunk now 

 migrate peripherally along the paths of the spinal nerves and 

 of the communicating rami (fig. 2) . 



At this stage, cells apparently become separated from the spinal 

 ganglia in considerable numbers. At the same time cells may be 

 traced from the mantle layer in the ventral part of the neural 

 tube, across the marginal veil, into the proximal parts of the ven- 

 tral nerve-roots. That cells migrate from the neural tube into 

 the ventral nerve-roots cannot be doubted. In many sections 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 11, NO. 3 



