SYMPATHETIC SYSTEM IN TURTLES 287 



elongated cell-aggregates lying along the dorsal surfaces of the 

 carotid arteries. Farther posteriorly the sympathetic anlagen 

 lie close to the dorso-lateral aspects of the aorta. From the 

 region of the suprarenals posteriorly scattered cell-groups may 

 be found as far ventrally as the ventral level of the aorta. 



The spinal nerves are at this stage large and conspicuous 

 fiber-tracts containing numerous accompanying cells which are 

 apparently migrating peripherally. Migration of medullary cells 

 into the ventral nerve-roots has probably reached its maximum at 

 about this stage. In nearly every section which passes through 





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CK 



Fig. 3 Transverse section through the anlage of the sympathetic trunk in an 

 eleven-day embryo of Thalassochelys caretta. X 210. Ao., aorta; CjB., communi- 

 cating ramus; Sp.N., spinal nerve; Sy. anlage of sympathetic trunk. 



the origin of a ventral nerve-root cells may be observed in the mar- 

 ginal veil, while in many sections complete lines of cells may be 

 traced from the mantle layer into the proximal part of the ventral 

 nerve-root. 



It may be of interest to note that in embryos of Thalasso- 

 chelys caretta cells may be traced from the ventral part of the 

 neural tube into the ventral nerve-roots more readily than in 

 embryos of Chelydra serpentina. This is probably due to the fact 

 that the former develop more slowly than the latter and that, 

 therefore, the migrant medullary cells linger for a longer period 

 at the point of exit from the neural tube. 



Fibers are now present in the dorsal nerve-roots connecting 

 the spinal ganglia with the neural tube. In a few sections cells 



