1s) 
Development of the Jugular Lymph Sacs. 21 
the circumference of the main vein, by further extension of the 
para-precardinal channel, while in a third group both the pre- 
cardinal vein and tributary 4 share more or less equally in fur- . 
nishing the elements which enter into the composition of the 
caudal division of the ventral veno-lymphatie plexus. 
Consequently an analysis of primary somatic tributary 4 and 
of the related precardinal segment, in reference to veno-lymphatic 
development, will give the clue to the interpretation of the various 
conditions encountered in the detailed study of the individual 
series. 
In the early stages the blood current of tributary 4 (4 in fig. 8), 
empties by a dilated terminal into the straight segment of the 
precardinal. 
Subsequently, a secondary channel (para-precardinal channel) 
is separated by condensation of plexiform areas of the main vein 
from the latter between the terminal of 4 and the promontory. 
This secondary “para-precardinal channel”’ (figs. 9 and 10) now 
drains the blood current of tributary 4, originally emptying into 
the precardinal, for the greater part directly into the promontory. 
The dilated and frequently fenestrated terminal of 4 and the para- 
precardinal channel, secondarily segregated from the main _pre- 
cardinal line, undergo veno-lymphatic development and form the 
structural basis for the further growth of the caudal division of the 
ventral veno-lymphatic plexus. 
In this process as stated above, tributary 4 may be largely 
taken up in the veno-lymphatic organization, or the latter may 
chiefly involve the main precardinal channel, leaving 4 to main- 
tain its somatic character. 
Hence in general the caudal division of the ventral veno-lym- 
phatic plexus presents the two main genetic types: 
1. Derivation from the ¢.ra-precardinal channel and the 
veno-lymphatic component of 4, leaving the dorsal somatic 
element of 4 as the first of the promontorial dorsal somatic 
branches. 
2. Derivation entirely from the para-precardinal channel, 
without involving any portion of tributary 4, which 
