328 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



larger vessels have valves at intervals to prevent the backflow of 

 the lymph, these valves often giving the vessels a nodulated appear- 

 ance. Proximally the lymph-vessels empty at two or more points 

 into the veins. The fluid portion of the lymph is derived in part by 

 osmosis from the walls of the blood capillaries, in part from the ali- 

 mentary canal. 



The development of the lymph-vessels has been traced mainly 

 in birds and mammals (chiefly in the latter), with fewer observations 

 on amphibia and other classes. Many points remain to be worked 

 out, there being considerable differences in the various accounts. 

 Apparently the process in its main features is as follows: 



Fig. 352. — 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^~', coccygeal veins; cd, Cuvierian duct; cv, cephalic 

 vein; dls, dorsal veno-lymphatic sinus; ej, ij, external and internal jugulars; pre, precava; 

 tk, thoracic duct; ul, primitive ulnar lymphatic; uva, anlage of ulnar vein; vis, ventral 

 veno-lymphatic sinus; 1-7, segmental vessels; lymphatic-forming tissue stippled. 



Near the junction of pre- and postcardinals on either side numer- 

 ous small diverticula are given off from the lateral side of these veins 

 (fig. 352, .4) . 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 con- 

 nexions are lost and the tubes are separated from the veins (fig. 352, .S) 

 forming an anterior cephalic duct, extending forward, parallel to the 

 jugular vein; an ulnar lynaphaiic duct destined to grow into the 

 fore limb; and, a little later, a thoracic duct grows back, parallel to 

 the postcardinal vein. All of these vessels are united near their 



