Development of the Jugular Lymph Sacs. 201 
In the early stages, the cephalic arch of the precardinal and the 
drainage area of its caudal or straight segment is situated lateral 
to the otocyst. Subsequently a network of anastomosing venous 
channels develops on the medial aspect of this structure and fin- 
ally forms the main intracranial channel. Tributaries draining 
this area in part empty directly into the newly formed channel and 
in part pass caudad to terminate in the dorso-medial aspect of the 
jugular promonotory caudal to the entrance of the primitive 
dorsal somatic branch 4 in fig 8. This change enables the caudal 
or straight portion of the precardinal to give up, in the majority 
of cases, the dorsal tributaries 1, 2, 3, and part or the whole of 4, 
and to contribute them toward the development of the veno- 
lymphatic plexuses. Consequently in certain stages the straight 
sepn ei t of the precardinal vein may appear partially denuded of 
dorsal somatic tributaries. Thesomatic vessels from 5S lo 7S or 8S, 
inclusive, descending caudad, enter the dorso-medial aspect of the 
promo.tory, while the dorso-lateral part of the promontory and 
precardinal, cephalad of the promontory, as well as precardinal 
tributaries 1, 2, 3 and 4, in whole or in part, are involved in the 
veno-lymphatic development. 
It is thus seen that the large size of the jugular promontory is 
due to the number and increasing caliber of the trunks, which 
progressively become confluent at this point, and that it represents 
so to speak, the slack of the early embryonic jugular system 
which permits, in the subsequent cardiac descent, the drawing 
out of the redundant promontory into the elongated vessel of 
later stages. 
4. Primitive Ulnar Segment of the Postcardinal Vein. 
Caudal to the entrance of dorsal somatic tributary 6S or 7S, 
(6S, 7S, fig. 8) into the jugular promontory, and to the origin 
ventrally of the duct of Cuvier, the postcardinal vein of the early 
stages, in the area receiving the seventh or the eighth, to the 
twelfth dorsal somatic branch (7S, 88S, fig.8), gives rise to a sec- 
ondary parallel channel, the primitive ulnar vein. This vessel 
develops along the dorsal circumference of the postcardinal by 
