290 ALBERT KTJNTZ 



process may be observed as early as the 64-hour stage. During 

 the fourth and the fifth day, after the spinal ganglia have be- 

 come well differentiated, a few cells may be observed migrating 

 from the dorsal part of the neural tube into the dorsal nerve- 

 roots (fig. 3, c. m. d. n. r.). It is probable that cells do not mi- 

 grate from the dorsal part of the neural tube in any considerable 

 numbers after the spinal ganglia have become differentiated. 

 Cell migration into the dorsal nerve-roots is probably only a tran- 

 sient process which takes part in the development of the spinal 

 ganglia. 



As the cells in the ventral nerve-roots migrate peripherally, 

 they mingle with similar cells which wander down from the spinal 

 ganglia. As there is no recognizable difference between the cells 

 which wander out from the spinal ganglia and those which migrate 

 peripherally along the ventral nerve-roots, it is impossible to 

 distinguish between the cells from these two sources after they 

 have passed beyond the point of union of the sensory and the 

 motor nerve-roots. As these cells migrate peripherally along the 

 spinal nerve-trunks, some of them deviate from the course of the 

 spinal nerves and migrate toward the sides of the aorta where they 

 become aggregated to give rise to the primary sympathetic trunks. 

 As migration proceeds, the cells which deviate from the course of 

 the spinal nerves no longer migrate into the primary sympathetic 

 trunks, but become aggregated at the median sides of the spinal 

 nerves to form the ganglionic enlargements which constitute 

 the anlagen of the secondary or permanent sympathetic trunks. 



His, Jr., has expressed the opinion that the elements composing 

 the primary sympathetic trunks are resolved into the ganglia and 

 nerves of the prevertebral and the peripheral sjmipathetic plexuses. 

 In view of the comparatively enormous development of the pre- 

 vertebral plexuses and of the ganglion of Remak in birds, this is ob- 

 viously the fate of the elements composing the primary sympa- 

 thetic trunks in the posterior region of the body. There is no evi- 

 dence, however, of the peripheral migration of cells from the prim- 

 ary sympathetic trunks in the anterior region. While there may 

 be some migration posteriorly along the primary sympathetic 

 trunks, it is more probable that most of the elements composing 



