130 Journal of Comparative Neurology and Psychology. 



Summary. 



(i) In the region between the vagus ganglion and the sec- 

 ond spinal nerve, the sympathetic arises in a comparatively sim- 

 ple and direct manner : cells, probably of epiblastic origin, scat- 

 tered in the mesoblast gradually become aggregated (Fig. 2, Sy. ) 

 to form a cellular cord. This is similar to the process in mam- 

 mals described by Paterson, except that he finds the cord is at 

 first entirely independent of any other structure, while in the 

 toad the cells of the sympathetic cord lie in contact with the fi- 

 bers of the first and second spinal nerves, from the earliest 

 stages. 



(2) In the region posterior to the second spinal nerve, the 

 origin of the sympathetic system is more complex. It appears 

 first as a small ridge of cells (Fig. i, Ri.) lying close along side 

 the aorta. The cells at the top of this ridge, as it becomes higher, 

 are differentiated to form the sympathetic cord (Fig's. 5 and 6, 

 Ri. and Sy.). Later, the ridge disappears entirely, leaving the 

 cord free, save its connections with the collateral sympathetic 

 and with the spinal nerves (Fig. 12, Co/. Sy. and R.). 



(3) The sympathetic ganglia and commissures arise di- 

 rectly from the sympathetic cord. The latter becomes enlarged 

 in the region of the nerves, forming the ganglia, while the por- 

 tions between the nerves become reduced, forming commissures, 

 which, immediately after metamorphosis, are composed largely 

 of nerve fibers. (Fig. 1 1 shows the enlargements and the 

 constricted portions.) These findings contrast somewhat with 

 the observations of Balfouk on elasmobranchs. where he finds 

 that the ganglia arise entirely independent of each other, the 

 commissures appearing later as outgrowths from the ganglia. 



(4) The rami communicantes arise in the toad in the same 

 manner as in elasmobranchs (Balfour). The cord (the gang- 

 lion in elasmobranchs) lies in contact with the nerve from the 

 very first (Fig. 4, Sy.). Later it gradually is removed from the 

 nerve, retaining, however, fibrous connections, which constitute 

 the ramus. The rami appear earliest in the mid-trunk region, 

 my preparations showing the first one in connection with the 



