Development of the Jugular Lymph Sacs. Zeit 
The dorsal veno-lymphatic plexus opens into the promontory 
at four points, separated by fenestrae of varying width. The 
anlage common to the primitive ulnar vein and its associated 
veno-lymphatic has separated from the postcardinal to a point 
about midway between the dorsal somatic tributaries 7S and 8S 
(7S and 8S in fig. 28), where it arches over the sixth segmental 
nerve to open into the dorso-lateral circumference of the promon- 
tory, and to become continuous in front with the dorsal veno-lym- 
phatic plexus. The external jugular anlage (figs. 27 and 28) still 
opens into the cardinal end of the duct of Cuvier, which has not, 
as yet, been incorporated in the promontory. 
The chief features of the left side of this embryo, as compared 
with the preceding earlier stages, may be enumerated as follows: 
1. More marked differentiation of cephalic arch from succeed- 
ing segment. 
2. Confluence and dilatation (with beginning fenestration) of 
terminals of dorsal precardinal tributaries 1, 2, 3. 
3. The establishment of a secondary promontorial connection 
for precardinal tributary 4. 
4. Clear differentiation of the jugular promontory from the 
straight portion of the precardinal and dilatation of the dorsal 
veno-lymphatic plexus. More marked separation from the post- 
-eardinal, by condensation of the capillary plexus, of the anlage 
of the primitive ulnar vein and veno-lymphatie. 
5. Assignment of three dorsal somatic tributaries (5S, 6S, 7S) 
to dorso-medial aspect of promontory (fig. 28). 
6. Differentiation of tributary 8S as first dorsal somatic branch 
of the postcardinal, caudad of primitive ulnar entrance. 
It is necessary to compare the lateral and medial aspects of the 
reconstruction in order to see the special features presented in 
this stage. They are shown in figs. 27 and 28. 
