CIRCULATORY SYSTEM. 



i 



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 



FIG. 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; c 1 - 5 , coccygeal veins; cd, Cuverian duct; cv, cephalic vein; dls, 

 dorsal veno-lymphatic sinus; ej, i], external and internal jugulars; prc, precava; th, thoracic 

 duct; ul, primitive ulnar lymphatic; uva, anlage of ulnar vein; vis, 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- 



