182 ALBERT KUNTZ 



municating rami cannot be traced among them. A little later 

 the fibrous communicating rami may be traced into these cell 

 aggregates (figs. 1 and 13). In the abdominal region some of 

 these fibers tend ventrally along the lateral aspects of the aorta 

 toward the regions in which the prevertebral plexuses arise (fig. 

 2). Cells of cerebrospinal origin are present, during early de- 

 velopment, both in the dorsal and ventral roots of the spinal 

 nerves as well as in the mixed nerve trunks and the paths of the 

 communicating rami. Migrant cells of medullary origin asso- 

 ciated with the fibers of the ventral root of a spinal nerve in an 

 embryo 9 mm. in length are illustrated in figures 3 A and 14. 

 Such cells are present in the motor nerve roots both within and 

 without the external limiting membrane and occasionally, as 

 illustrated microphotographically in figure 14, cvr, an individual 

 cell may be observed in the process of passing through this mem- 

 brane. Beyond the junction of the dorsal and ventral roots of 

 the spinal nerves the cells of medullary and ganglionic origin 

 cannot be distinguished from each other. As these cells advance 

 peripherally, some of them enter the sympathetic primordia to 

 give rise to sympathetic neurones. 



Some of the more recent investigators, including the present 

 writer, have expressed the opinion that many of the cells which 

 enter the sympathetic primordia migrate peripherally in advance 

 of the growing nerve fibers. The early migration of sympathetic 

 cells in human embryos was emphasized by Streeter ('12), who 

 expressed the opinion that the majority of the cells which enter 

 the primordia of the sympathetic trunks advance toward the 

 aorta before fibers are present in the communicating rami; con- 

 sequently, he described the early communicating rami as cellu- 

 lar strands. Ganfini ('17)^ also described cellular communicating 

 rami in early mammalian embryos (guinea-pig and pig) which 

 later give way to the fibrous rami. As is well known, the distal 

 portions of growing nerve fibers are not brought out clearly by 

 the ordinary staining processes; however, as illustrated in figures 

 1 and 13, fibers are present in the communicating rami relatively 



^ I am indebted to my student, Mr. JoseZozaya, for reading Ganfini's and other 

 Italian papers. 



