SYMPATHETIC SYSTEM IN MAMMALS 235 



In embryos 13 mm. in length, the cell-aggregates lying along 

 the ventro-lateral surfaces of the aorta have become more pro- 

 nounced. The greatest development occurs in the region of the 

 suprarenals. At a few points, fibers may be traced from the 

 sympathetic trunks into the anlagen of the prevertebral plexuses 

 (fig. 12, f.pv.). In the region of the coeliac plexus, fibers have 

 advanced farther peripherally and may be traced for a short 

 distance into the mesentery. The anlagen of the abdominal 

 aortic plexus have developed into a loose network which com- 

 pletely surrounds the aorta ventrally. 



In embryos 16 mm. in length, the prevertebral plexuses are 

 becoming more distinct and more fibrous. The sympathetic 

 trunks are more sharply limited ventrally, except in the region 

 of the suprarenals. At this point there is still a continuous line 

 of sympathetic cells extending from the sympathetic trunks into 

 the cell-aggregates associated with the suprarenals. 



(b) Histogenetic relationships. — The cells constituting the an- 

 lagen of the prevertebral plexuses show all the characters of the 

 cells present in the sympathetic trunks. Continuous lines of 

 cells may be traced from the latter into the former. The prever- 

 tebral plexuses, therefore, stand in direct genetic relationship 

 to the sympathetic trunks. Mitotic figures occur occasionally 

 along the courses of migration from the sympathetic trunks as 

 well as in the anlagen of the prevertebral plexuses. Doubtless, 

 a goodly number of the cells taking part in the development of 

 the prevertebral plexuses arise by the mitotic division of "in- 

 different" cells along the courses of migration. The develop- 

 ment of the prevertebral plexuses is, therefore, entirely analogous 

 with the development of the sympathetic trunks. 



III. VAGAL SYMPATHETIC PLEXUSES 



(a) Introductory. — Under the term vagal sympathetic plex- 

 uses, we shall consider those plexuses, usually regarded as sym- 

 pathetic, which are directly related to the vagi; viz., the cardiac 

 plexus and the sympathetic plexuses in the walls of the visceral 

 organs. 



