18() ALBERT KUNTZ 



throughout the thoracic and abdominal regions. The conden- 

 sations which result in three ganglionic masses in the cervical 

 region also have become apparent. Fibers may now be traced 

 from the ujiper ends of the superior cervical ganglia cephalad 

 along the internal carotid arteries. These ganglia are not sharply 

 limited at their upper extreiTiities, and apparently some cells 

 advance along the fibers of the internal carotid nerves. The 

 majority of these cells probably become incorporated in the 

 plexuses on the internal carotid arteries. In embryos 20 to 22 

 mm. in length the sympathetic trunks have assumed a relation- 

 ship to the vertebral condensations. The ganglia are quite 

 sharply delimited and the fibrous rami between them are rela- 

 tively free from cells. 



PREVERTEBRAL PLEXUSES 



In the upper abdominal region of an embryo 6 mm. in length 

 (no. 676) a few cells may be traced from the primordia of the 

 sympathetic trunks ventrally along the lateral aspects of the 

 aorta. In a few segments of this embryo and in embryos which 

 are somewhat farther advanced some of the fibers of the com- 

 municating rami tend ventrally along the paths of these cells 

 (fig. 2). In embryos which are somewhat older, fibers which 

 arise from cells in the primordia of the sympathetic trunks also 

 grow ventrally into the region in which the ganglia of the pre- 

 vertebral plexuses arise. During this interval cells become sepa- 

 rated from the ventral borders of the primordia of the sympathetic 

 trunks throughout the abdominal region and advance ventrally 

 to give rise to the ganglia of the prevertebral plexuses. At the 

 same time other cells which are associated with those fibers of 

 the communicating rami which turn ventrally without passing 

 through the ganglionic masses at the dorsolateral aspects of the 

 aorta advance into the prevertebral plexuses more or less di- 

 rectly from the spinal nerves. In embryos 9 to 10 mm. in length 

 the aggregates of sympathetic cells lying along the ventral and 

 ventrolateral aspects of the abdominal aorta have assumed con- 

 siderable size. In an embryo 10.1 mm. in length (no. 623) these 



