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: 



"1. 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, 

 Iymphoglandula; N.III, 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.J., vena femoralis; V.j.i., vena 

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

 V.I.S., vasa lymphatica superficialis; 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 



