CIRCULATORY SYSTEM. 303 
Near the junction of pre- and postcardinals on either side numerous 
small diverticula are given off from the lateral side of these veins (fig. 
309, A). These diverticula unite with each other, forming small 
tubes parallel to the parent vessels and united to them for a time at 
numerous points where the budding took place. Later these connex- 
ions are lost and the tubes are separated from the veins (fig. 309, B) 
forming an anterior cephalic duct, extending forward, parallel to the 
jugular vein; an ulnar lymphatic duct destined to grow into the fore 
limb; and, a little later, a thoracic duct grows back, parallel to the 
Fic. 309.—Early development of the lymph vessels in the cat, after McClure and 
Huntington. A, in 6.5 mm. embryo; B, in 10.5 mm. embryo; C, definitive stage; D, 
diagram of developing diverticula of chick which are to form lymph heart, based on Sala. 
ac, anterior cardinal vein; ¢ *-*, coccygeal veins; cd, Cuverian duct; cv, cephalic vein; dls, 
dorsal veno-lymphatic sinus; ej, 7j, external and internal jugulars; pre, precava; th, thoracic 
duct; ul, primitive ulnar lymphatic; uva, anlage of ulnar vein; vls, ventral veno-lymphatic 
sinus; 1-7, segmental vessels; lymphatic-forming tissue stippled. 
postcardinal vein. All of these vessels are united near their point of 
origin by a large sinus, the jugular lymph sac (fig. 309, C). Later 
the lymph sac reestablishes communication at one or two points in the 
subclavian-jugular region with the vein. 
The conditions at the posterior part of the body are less certainly 
known (fig. 309, D). In this region a cistern of chyle (a mesenterial 
lymph sac) and a posterior lymph sac develop in close connexion 
with the postcava in the region of the nephridial organs, and it is pos- 
sible that a portion of the thoracic duct grows forward from the cis- 
tern of chyle, while other vessels grow into other regions. Later the 
primitive trunks thus outlined give off branches which gradually ex- 
