THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



277 



Two pairs of lymph sacs develop, the jugular lymph sacs in the cervical and 

 upper thoracic regions in relation to the precardinal veins and the caudal lymph 

 sacs in the lumbar region in relation to the postcardinal veins. Comparatively 

 little is known about these structures in the human embryo, but the develop- 

 ment of the jugular lymph sacs in the domestic cat has been worked out with 

 great care and thoroughness by Huntington and McClure. 



As stated by these authors, "the 

 primary principles underlying the de- 

 velopment of the jugular lymph sac are: 

 (i) the development of a secondary 

 channel parallel to the embryonic pre- 

 cardinal vein and the Cuvierian end of 

 the post cardinal; (2) the association 

 with this secondary channel of a certain 

 number of dorsal precardinal tributaries, 

 and (3) the separation of these two sets 

 of venous elements, which have been 

 termed veno-lympkatics, from the main 

 venous channels, and their subsequent 

 conversion into the definite jugular lymph 

 sacs by a process of growth and fusion." 

 A number of venous channels along the 

 dorsal side of the straight segment of 

 the precardinal and the cephalic end of 

 the postcardinal vein enlarge and begin 

 to coalesce. The first four dorsal somatic 

 tributaries of the straight segment of the 

 precardinal become dilated and fuse 

 together. These two groups of venous 

 channels become incorporated into a 

 single group and, by a further process 

 of coalescence of their component ele- 

 ments, form a sac-like structure. This then evacuates its blood contents into 

 the large veins and becomes completely separated from the venous system. 

 Subsequently it establishes new and permanent communication with the venous 

 system either in the angle between the external and internal jugular veins — 

 common jugular tap, or between the subclavian and common jugular — jugulo- 

 subclavian tap, or at both points. 



The systemic lymphatic trunks join the lymph sacs and through them open 

 into the veins at the points mentioned above, the sacs thus serving as portals 

 of entry for the main systemic lymphatics into the venous system. The sacs 



Fig. 253. — Diagram showing network of 

 lymphatic vessels in skin of pig embryos. 

 Sabin. 



Area marked A shows extent of network in 

 an embryo of 18 mm.; B, in embryo of 

 20 mm.; C, in embryo of 30 mm.; D, in 

 embryo of 40 mm. 



