SYMPATHETIC SYSTEM IN TURTLES 295 



part of the neural tube in embryos of the turtle also answer to the 

 description of the 'indifferent' cells of Schaper. They are char- 

 acterized by very little cytoplasm and by large rounded or elon- 

 gated nuclei showing a delicate chromatin structure. In the sym- 

 pathetic anlagen some of these cells early develop protoplasmic 

 processes and may, therefore, be recognized as neuroblasts. 

 Although neuroblasts have not infrequently been observed in the 

 spinal and in certain of the cranial nerves in vertebrate embryos, 

 I was not able to observe cells with distinct protoplasmic processes 

 along the paths of migration of the cells giving rise to the sympa- 

 thetic nervous system in embryos of the turtle. 



The great majority of the cells present in the mantle layer in the 

 neural tube in embryos of the turtle answer to the description 

 given above for the cells which migrate peripherally. There can 

 be no doubt, therefore, that the cells which take part in the devel- 

 opment of the sympathetic trunks have the same genetic relation- 

 ships as the cells which give rise to the neurones and to the neu- 

 rolgia cells in the central nervous system. Mitotic figures in 

 their various phases occur frequently all along the paths of 

 migration and in the sympathetic anlagen. We are not to sup- 

 pose, therefore, that ail the cells taking part in the development 

 of the sympathetic trunks actually migrate as such from their 

 sources in the cerebro-spinal nervous system. Doubtless, many 

 arise by " the mitotic division of ' indifferent ' cells along the 

 course of migration as well as in the sympathetic anlagen. 



Prevertebral plexuses 



At the close of the eleventh day of incubation of embryos of 

 Thalassochelys caretta, after the anlagen of the sympathetic 

 trunks have begun to break up and to become somewhat scat- 

 tered, cells may be observed wandering ventrally from the anla- 

 gen of the sympathetic trunks in the entire region from the 

 suprarenals posteriorly. At the close of the thirteenth day, 

 these cells have become aggregated into groups of considerable 

 size lying along the ventro-lateral aspects of the aorta. These 

 cell-groups constitute the anlagen of the prevertebral plexuses. 



