Development of the Jugular Lymph Sacs. 257 
dorsal somatic branch (4S) has been separated from tributary 4 
and the para-precardinal channel, which now opens into the dorso- 
medial surface of the promontory, where it forms the first of a 
series of promontorial somatic tributaries (4S, fig. 40). The 
veno-lymphatic component of tributary 4 (4VL, fig. 40) which 
has severed its connections with the precardinal and the para-pre- 
cardinal channel, now occupies a dorso-lateral position with respect 
to the precardinal, and lies in line with the dorsal veno-lymphatic 
plexus and the veno-lvmphatics derived from tributaries 1, 2 and 
3, with which it will subsequently fuse to form the dorsal veno- 
lymphatic arch in fig. 14. The para-precardinal channel still 
communicates with the precardinal and the promontory, being 
separated from the former by the fenestra Y. The para-pre- 
cardinal channel on the left side of this embryo constitutes the sole 
anlage of the caudal division of the ventral veno-lymphatic plexus, 
precardinal tributary 4 per se not contributing to its formation. 
As the result of a caudal extension which involved the proximal 
ends of the postcardinal and the duct of Cuvier, the promontory 
has increased in size in an antero-posterior direction. In con- 
sequence of this caudal extension, the duct of Cuvier joins the 
posteardinal further caudad than in the earlier stages and no 
longer receives the anlage of the permanent external jugular vein, 
which now opens into the ventro-lateral circumference of the 
promontory. The promontory has not yet assumed the rounded 
and sac-like appearance in this embryo which is so characteristic 
of this area in certain other members of this group. 
The anlage common to the primitive ulnar vein and primitive 
ulnar veno-lymphatic has completely separated from the post- 
cardinal. After arching over the sixth spinal nerve (SF.N.V1Z, 
fig. 40) it opens into the dorso-lateral circumference of the prom- 
ontory, contiguous to the latter’s point of confluence with the 
sixth dorsal somatic tributary (6S). Anterior to its union with 
the promontory it becomes continuous with the dorsal veno- 
lymphatic plexus. 
The most marked advance is shown in this embryo in the dors9- 
lateral circumference of the promontory and of the arch of the 
primitive ulnar vein. This segment of the primitive venous 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 10, NO. 2. 
