SYMPATHETIC SYSTEM TN MAMMALS 233 



Among the "indifferent" cells of Schaper, cells are occasionally 

 found which are characterized by large rounded or elongated 

 nuclei showing a well defined nucleolus and very little chromatin 

 structure, and a considerable quantity of cytoplasm which is 

 usually drawn out to a point at one side (fig. 10). These cells 

 are obviously the "neuroblasts" of Schaper. They are few in 

 number, but occur all along the path of migration of the sympa- 

 thetic cells. I have observed them in the ventral nerve-roots 

 both inside and outside the external limiting membrane, in the 

 spinal nerves, in the communicating rami, and in the anlagen of 

 the sympathetic trunks. 



The histogenetic relationships of the cells taking part in the 

 development of the sympathetic trunks will be considered further 

 in section V. The facts of importance at this point are that 

 cells which are endowed with a capacity to develop into neurones, 

 migrate peripherally from the neural tube and the spinal ganglia, 

 and that some of these cells migrate into the anlagen of the sym- 

 pathetic trunks. These facts establish a direct genetic relation- 

 ship between the sympathetic and the central nervous system. 

 We are not to suppose, however, that all the cells taking part in 

 the development of the sympathetic trunks actually migrate as 

 such from their sources in the neural tube and the spinal ganglia. 

 Doubtless, many arise by the mitotic division of "indifferent" 

 cells along the course of migration. The sources of these migrat- 

 ing elements are, therefore, sufficient to account for all the cells 

 which take part in the development of the sympathetic trunks 

 and the sympathetic plexuses genetically related to them. 



II. PREVERTEBRAL PLEXUSES 



(a) Development. — In embryos 10 mm. in length, the anlagen 

 of the prevertebral plexuses may be recognized as small cell- 

 aggregates lying along the ventro-lateral surfaces of the aorta in 

 the dorsal and the lumbar regions. In these regions the sympa- 

 thetic trunks are not sharply limited ventrally. Cells become 

 separated from their ventral margins and migrate ventrally along 

 the sides of the aorta (fig. 11, c.m.pv.). In the region of the 



