DEVELOPMENT OF THE JUGULAR LYMPH SAC 491 

 SUMMARY 



Venous islands and channels appear in the mesenchymal tissue 

 dorsolateral to the precardinal and postcardinal veins near their 

 junction to form the duct of Cuvier. These islands and channels 

 constitute what has been called in this article a lateral group of vas- 

 cular elements in contradistinction to a mesial group which forms 

 the dorsal somatic (intersegmental) veins, although both groups 

 are at first rather closely associated. Some of the lateral elements 

 open into the cardinal veins (figs. 1, 2 and 3). The elements of 

 the lateral group enlarge and new capillaries are added until, 

 eventually, the group as a whole constitutes an extensive plexus 

 of veno-lymphatic channels, the plexus also becoming distinctly 

 separated from the more mesially situated dorsal somatic veins 

 (figs. 4 and 5). The veno-lymphatics, by progressive enlargement 

 and coalescence, then develop into a distinct sac-like structure 

 and in the meantime become completely cut off from the venous 

 system, thus forming the prelymphatic sac (figs. 6 and 7). The 

 prelymphatic sac subsequently acquires secondary communica- 

 tion with the venous system through a tap situated on the dorso- 

 mesial side of the precardinal (jugular) vein at the level of the sub- 

 clavian, and thus becomes the jugular lymph sac (figs. 8, 9 and 10). 



