THE LYMPHATIC SYSTEM 



287 



In embryos of 30 mm. the thoracic duct has developed, connecting the left 

 jugular sac with the retroperitoneal sac and receptaculum chyli (Fig. 279). Ac- 

 cording to Lewis, the thoracic 

 duct arises from the union of 

 several detached lymphatics de- 

 rived originally from the veins. 

 The receptaculum chyli "is a 

 secondary enlargement dorsal to 

 the aorta." In later stages the 

 lymph-sacs are replaced by plex- 

 uses of lymphatic vessels which 

 grow to the head, neck, and arm 

 from the jugular sacs; to the hip, 

 back and leg from the posterior 

 sacs, and to the mesentery from 

 the retroperitoneal sac. Valves 

 are developed in the lymphatic 

 vessels from folds of their endo- 

 thelium. 



Huntington and McClure be- 

 lieve that the isolated lymphatic 

 spaces are derived independently from 

 the mesenchyma. Lewis (in Amer. 

 Jour. Anat., 1905, vol. 5, pp. 95-120) 

 rejects this view for the following 

 reasons: 



"i. The lymphatic spaces do 

 not resemble mesenchyma. 



"2. After being formed the 

 lymphatics increase like blood-vessels 

 by means of blind endothelial sprouts 

 and not by connecting with intercel- 

 lular spaces. 



"3. In early embryos detached 

 blood-vessels may be seen without 

 proving that blood-vessels are mesen- 



FIG. 279. Flat reconstruction of the primitive lym- 

 phatic system in a human embryo 30 mm. long, Mall 

 collection, No. 86, X about 5.4. Cc, cisterna chyli; Lg, 

 lymphoglandula; N. Ill, N. IV, and N. V, Nn. cervi- 

 cales; S.l.jug., saccus lymphaticus posterior; S.l.s., sac- 

 cus lymphaticus subclavius; V.c., vena cephalica; V.c.i., 

 vena cava inferior; V.f., vena femoralis; V.j.i., vena 

 jugularis inferior; V.l.p., vasa lymphatica profunda; 

 V.I.S., vasa lymphatica superficial; V.r., vena renalis; 

 v.s., vena sciatica; V.u. (p.), vena ulnaris (primitiva) 

 (Sabin). 



chymal spaces. 



"4. The endothelium of the lymphatics is sometimes seen to be continuous with that 

 of the veins." 



The Lymph Glands. These are developed first as plexuses of lymphatics 

 in connection with an artery and vein. The first pair are formed in the axillary 



