SYMPATHETIC SYSTEM IN BIRDS 303 



accomplished by the peripheral growth of the axones alone or 

 whether cells may also migrate peripherally, independently of 

 the growth of the axones. My observations do not enable me to 

 offer any adequate explanation of the process by which the cells 

 giving rise to the sympathetic nervous system are carried periph- 

 erally from the cerebro-spinal system. The growing nerve-fibers, 

 doubtless, constitute an important factor in the peripheral trans- 

 portation of these elements. They are not sufficient, however, 

 to account for the entire process alone. Nor is the presence of 

 nerve-fibers absolutely necessary to the peripheral migration of 

 sympathetic cells. In embryos of both birds and mammals, 

 cells may be traced from the spinal nerves into the anlagert of the 

 sympathetic trunks before fibers are present in the communicat- 

 ing rami. Likewise, cells migrate ventrally from the sympathetic 

 trunks into the anlagen of the prevertebral plexuses before post- 

 ganglionic fibers appear. 



Held ('09) and Marcus ('09) have recently taken exception to 

 Froriep's views concerning the origin of the cells giving rise to 

 the sympathetic nervous system. Held has attempted to show, 

 for the entire vertebrate series, that the cells present in the motor 

 nerve-roots play no part in the development of the sympathetic 

 nervous system. He still regards the sympathetic system as an 

 offshoot from the spinal ganglia. In the light of the present in- 

 vestigation, such a position is untenable. My preparations show 

 conclusively that medullary cells migrate into the motor nerve- 

 roots in considerable numbers. These cells migrate peripherally 

 along the spinal nerves just as certainly as do the cells which 

 wander down from the spinal ganglia. Inasmuch as the great 

 majority of the cells migrating peripherally along the spinal nerves 

 are cells of an indifferent character, there is no reason to suppose 

 that the cells which wander down from the spinal ganglia give rise 

 to sympathetic neurones, while those which migrate from the 

 neural tube along the fibers of the motor nerve-roots do not. 



Marcus has attempted to show that the cells which Froriep 

 observed in the ventral roots of the spinal nerves do not wander 

 out from the neural tube, but migrate thither from the neural 



