HISTOGENESIS OF SYMPATHETIC SYSTEM 13 
have advanced from the intermediate portion of the wall of the 
neural tube. These observations strongly suggest that the 
normal contribution of cells from the spinal ganglia to the pri- 
mordia of the sympathetic trunks is relatively small. 
Embryos of the frog from which a strip of tissue including the 
neural crests and the dorsal portion of the neural tube was re- 
moved dorsally before the neural tube was completely closed and 
which were killed three to five days later show complete absence 
of spinal ganglia and dorsal nerve-roots throughout the segments 
involved. Ventral nerve-roots accompanied by cells of medul- 
lary origin, as well as small primordia of the sympathetic trunks, 
are present in all the segments in which a considerable portion 
of the neural tube was left intact. The number of cells in the 
primordia of the ganglia of the sympathetic trunks varies with 
the size of the remnant of the neural tube in the corresponding 
segment. As far as the study of the development of the sym- 
pathetic trunks was pursued in these embryos, the findings cor- 
roborate those in the operated embryos of the chick. Embryos 
of the frog are less favorable for studies of this character than 
embryos of the chick by reason of the small number of sym- 
pathetic elements normally present. Inasmuch as the embryos 
of the frog included in this experimental series illustrate no 
phase of the histogenesis of the sympathetic trunks which is 
not equally well or better illustrated by the embryos of the 
chick,’ the conditions which obtain in the former need not be 
set forth in greater detail at this time. 
The prevertebral plexuses 
The direct genetic relationship of the prevertebral sympathe- 
tic plexuses to the sympathetic trunks is generally conceded, 
except by these who advocate the theory of the mesodermal ori- 
gin of the sympathetic nervous system. As pointed out by the 
writer in earlier studies, the majority of the cells which enter the 
primordia of the prevertebral plexuses are cells which become 
displaced ventrally from the primordia of the sympathetic trunks. 
Other cells which advance toward the latter primordia along the 
