Development of the Jugular Lymph Saes. 2i7 
The dorso-lateral circumference of the promontory and_post- 
cardinal carries a plexiform net work which, along the lines above 
indicated (page 205), condenses by fusion and enlargement of the 
interreticular spaces into a dorso-lateral component of the main 
channel, representing the anlage of the dorsal veno-lymphatic 
plexus. The result of this process is shown in figs.8, 9and 10, in 
which the dorsal veno-lymphatic plexus appears as a dilated 
sac, extending cephalo-laterad from the promontory over the 
junction with the precardinal and separated from the main sys- 
temic channel by a linear series of fenestree. This plexus subse- 
quently develops into the dorsal division of the general veno- 
lymphatic plexus, and forms, in later stages, after amalgamation of 
the veno-lymphatic components into the single jugular lymph sae, 
the portion of the latter to which we have applied the term ‘‘ sub- 
clavian approach. ”’ 
Figs. 8, 9, 10, 11 and 12 illustrate the general character and pro- 
gressive growth of the dorsal plexus in the early veno-lymphatic 
stages, and fig. 12 the drawing out of the interfenestral areas into 
elongated channels, preparatory to the separation of the dorsal 
plexus from the promontory. 
The secondary capillary network in which the dorsal plexus takes 
its origin from the main venous channel, extends caudad along 
the promontory and postcardinal vein beyond the confines of the 
dorsal veno-lymphatic plexus proper. 
Thus the linear series of fenestrae shown in figs. 9 and 10, by 
enlargement and confluence, finally separate a secondary channel 
from the main posteardinal vein, which retains its cephalic con- 
nection with the promontory near the entrance of the sixth or 
seventh dorsal somatic tributaries (6S, and 7S, in figs. 11, 12 and 
13). This secondary parallel vessel is the primitive ulnar vein, 
serving as a temporary drainage channel for the anterior limb- 
bud. Subsequently, in repetition of its own genesis from the post- 
cardinal vein, a plexiform capillary network, extending along the 
dorsal circumference of the primitive ulnar vein, condenses into 
a parallel vessel which terminates in the caudal part of the dorsal 
veno-lymphatic plexus. Hence we have in many of our diagrams 
designated the area involved as the ‘‘anlage common to the primi- 
tive ulnar vein and the primitive ulnar veno-lymphatice. ”’ 
