202 George S. Huntington and Charles F. W. McClure. 
condensation of a peri-venous plexiform capillary network. The 
impetus to its formation is apparently given by the return circu- 
lation from the anterior limb-bud through the marginal vein. 
This vessel, in its rudiments, at first drains into the somatic 
branches of the umbilical vein. With the diversion of the um- 
bilical to the hepatic circulation and with the resulting transferal 
of the somatic return to the posteardinal line, the primitive ulnar 
appears as a secondary derivative from the latter, serving, before 
the establishment of the subclavian vein, as the main channel 
returning the blood from the anterior extremity. As such, it 
develops from the capillary network surrounding the postcardinal 
vein by condensation of the plexiform reticulum along the dorsal 
circumference of the main vessel, and opens into the promontory 
at the level of the sixth dorsal somatic tributary (6S in series 
106, 9 mm. embryo, fig. 40). 
The series of fenestre in the postcardinal vein (fig. 8), leading 
up to the jugular promontory, indicate the line along which sub- 
sequently the primitive ulnar vein, by further extension and con- 
fluence of the fenestrae, continues to separate from the postcardinal 
vein proper, until only its cephalic termination into the dorso- 
lateral aspect of the jugular promontory is retained as its definite 
point of connection with the embryonic venous system. 
The part played by this vessel in the early drainage of the ante- 
rior limb-bud, in the subsequent development of the dorsal 
veno-lymphatic plexus, and its final relation to the adult type of 
lymphatic organization, makes it one of the most important fac-_ 
tors in the evolution of the definite lymphatic system. The de- 
tailed history of this vessel may therefore be properly considered 
under the separate headings, dealing with the individual stages. 
Il. ANALYSIS OF DEVELOPMENTAL STAGES IN THE FORMATION 
OF THE JUGULAR LYMPH SACS. 
As already stated, the jugular lymph sac, or anterior lymph 
heart, develops in the embryo of the Domestic Cat as a direct 
derivative of the early embryonic venous system. It arises from 
certain of the dorsal tributaries of the precardinal and from the 
