Development of the Jugular Lymph Sacs. 279 
region pass between the primitive ulnar anlage and the main 
systemic venous channel. 
Series 101, 10°" Embryo 
Reconstruction of right side, 
Lateral aspect, fig. 50 
5) 
The right side of this embryo (fig. 50) offers an interesting com- 
parison with the veno-lymphatic development found on the left 
side (fig. 49). 
A. Dorsau Veno-LympHatic Plexus. 
The dorsal sac presents the same forward extension and multi- 
fenestrated appearance as that on the left side, but its relations 
to the promontory and to the straight segment of the precardinal 
differ in a number of important features from those of the oppo- 
site side. 
1. On the right side (fig. 50) the dorsal plexus is plainly separ- 
ated into two portions, one derived from the promontory proper, 
the other from the arch of the primitive ulnar vein. 
The posterior and smaller part of the general dorsal veno-lym- 
phatic plexus is not connected, as it is on the left side, with the 
larger anterior part. 
2. The anterior end of the dorsal plexus has established a con- 
nection, near the level of the cephalic arch, with the straight seg- 
ment of the precardinal (Anterior Tap of Evacuation). This — 
connection results from the fusion between the anterior end of 
the dorsal plexus and the veno-lymphatic component of a dorsal 
tributary of the straight segment of the precardinal which has 
retained its primary connection with the latter, or, which, in case 
of early multiple precardinal terminals, has continued to develop 
one of these, thus accounting for the difference in the level of the 
first precardinal veno-lymphatiec tap observed on the two sides of 
this embryo in reference to the level of the spinal nerves. 
On the left side the abandonment of the early precardinal termi- 
nal of tributary 1, together with the retention of the terminals of 
tributaries 2 and 3, would produce the relations to the segmental 
nerves found on this side of the embryo, while the relations of the 
