SYMPATHETIC SYSTEM IN BIRDS 293 



cells present in the spinal nerves are obviously neuroblasts. Such 

 cells have frequently been observed outside the neural tube and 

 the spinal ganglia. Cajal ('08) described cells which he recog- 

 nized as nerve cells in the bipolar phase, in the motor roots of the 

 spinal nerves and in certain of the cranial nerves in the chick. 

 These cells, he believes, correspond to the real motor cells in the 

 neural tube. Mitotic figures occur occasionally all along the 

 course of migration and in the sympathetic anlagen. We are 

 not to suppose, therefore, that all the cells taking part in the de- 

 velopment of the sympathetic trunks actually migrate as such 

 from their sources in the cerebro-spinal nervous system. Doubt- 

 less, many arise by the mitotic division of " indifferent " cells 

 along the course of migration. 



2. Prevertebral plexuses 



The prevertebral plexuses are derived directly from the pri- 

 mary sympathetic trunks. They arise about the middle of the 

 fourth day (108-hour stage), as cell-aggregates lying along the 

 ventro-lateral aspects of the aorta from the suprarenal bodies 

 posteriorly. In this region the primary sympathetic trunks are 

 not sharply limited ventrally. Sympathetic cells may be traced 

 from the latter directly into the anlagen of the prevertebral plex- 

 uses. In the sacral region, the aorta is soon completely surrounded 

 ventrally by a ring of loosely aggregated sympathetic cells (fig. 

 5, hyp.). 



The cell-aggregates constituting the anlagen of the prevertebral 

 plexuses increase very rapidly. At the close of the fourth day 

 (120-hour stage), these plexuses have become well established. 

 Distinct lines of cells may be traced from the primary sympathetic 

 trunks directly into cell-aggregates of considerable size lying 

 along the median sides of the suprarenals, and wandering sym- 

 pathetic cells may be observed all along the sides of the aorta from 

 the suprarenals posteriorly. The limits of the anlagen of the 

 several prevertebral plexuses cannot be determined at this stage. 

 Traces of one or the other of these plexuses are not wanting in 

 any transverse section in this entire region. 



