Development of the Jugular Lymph Sacs. 209 
Thus, while the genetic principles involved in the formation of 
the two divisions of the ventral veno-lymphatic plexus are iden- 
tical, it will prove more serviceable to distinguish between the two 
in the preliminary analysis, because certain special features in the 
relation of dorsal somatic tributary 4 to the precardinal vein and 
the promontory modify the picture of the development of the 
eaudal portion of the ventral veno-lymphatic plexus and serve to 
distinguish it in early stages from the cephalic division of the same. 
We will therefore first consider the development of the cephalic 
or anterior division of the ventral veno-lymphatic plexus, then 
that of the caudal division, thirdly that of the dorsal veno-lym- 
phatie plexus, and finally take up the mutual relations of these 
three divisions prior to their fusion into the common veno- 
lymphatic sac. 
This can best be accomplished by following the veno-lymphatic 
development on the hand of the series of diagrams shown in 
figs. 8 to 13 which, with the exception of fig. 11A, represent in 
schema lateral views of the reconstructions of the left side in 
profile, but which answer equally well for the conditions found on 
the right side. Fig. 11A represents a dorsal view of fig. 11 in the 
region of the jugular promontory. 
1. Development of the Cephalic or Anterior Division of the 
Ventral Veno-Lymphatic Plexus. 
The formation of this portion of the ventral veno-lymphatic 
plexus can be analyzed by considering the two main types of 
development which it exhibits, with the understanding that in 
individual instances one or the other of these types may predomi- 
nate, or that they may share equally inthe production of this 
portion of the ventral plexus. 
First Mode of Development. In this type the cephalic part of 
the ventral veno-lymphatic plexus is largely developed from the 
three anterior dorsal somatic branches of the precardinal vein. 
The early precardinal tributaries 1, 2, and 3 in fig. 8, enlarge 
at their terminals in a funnel-shaped manner, and these expanded 
bases present a more or less distinct plexiform or fenestrated ap- 
pearance (1, 2, and 3 in fig. 10). 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 10, NO. 2. 
