Development of the Jugular Lymph Sacs. 269 
The dorsal somatic component of tributary 4 (4S) is entirely 
detached from the veno-lymphatic plexus, and enters, as the first 
of the somatie series, the dorso-medial aspect of the promontory. 
II. Dorsat VeENo-LYMPHATIC PLEXUS. 
This is a much dilated and fenestrated sac which lies dorso- 
lateral to the promontory, and which has been derived from the 
same. It extends forward for some distance in front of the prom- 
ontory and communicates with the latter by a narrow channel 
which opens into the promontorial end of the primitive ulnar 
vein (Tap C). The presence of a single point of communication 
between the dorsal plexus and the promontory at the primitive 
ulnar entrance corresponds to the conditions already described 
for the left side of this embryo (Tap C, fig.44). The relations of the 
the dorsal plexus to the thyo-cervical artery are also the same and 
need, therefore, no further mention. 
The primitive ulnar vein makes a distinct arch in its terminal 
portion and receives a long cephalic tributary which follows the 
dorsal border of the dorsal veno-lymphatic plexus. The primitive 
ulnar veno-lymphatic was not fully reconstructed on this side of 
the embryo but presents about the same conditions as on the left 
side. 
In addition to the three primary veno-lymphatic plexuses already 
described, a number of detached veno-lymphatic elements are 
met with on the right side of this embryo. These are four in num- 
ber and are labelled VL’ in fig. 46. The significance and subse- 
quent fate of these detached veno-lymphatics has already been 
discussed on a preceding page (page 259) and therefore need not 
be further considered here. 
The medial aspect of this reconstruction, which is not figured, 
like that of the left side (fig. 45), offers a typical instance of the 
serial arrangment of dorsal somatic tributaries, beginning with 
4S, which enter the dorso-medial aspect of the promontory. 
As this embryo serves as a connecting link between the early 
and later veno-lymphatic stages, it may be well to summarize 
the principal features which it illustrates. 
