The Lymphatic System in Human Embryos. 49 



of the American Journal of Anatomy, which show complete injec- 

 tions of the skin for each stage, it will be seen that the lymphatics in- 

 vade non-lymphatic areas, even in the last figure of the series where all 

 of the systems have anastomosed over the body there is a marked 

 non-lymphatic area over the top of the head as well as over the feet. 

 In pigs longer than 5.5 cm., it is difficult to obtain such extensive 

 injections because valves begin to develop and tend to make the lymph 

 flow from periphery to centre. During this early period of the 

 spread of the lymphatics over the body there are no valves whatever, 

 v/hich accounts for the wide extent of the injection shown for a pig 

 5.5 cm. long. 



To trace these vessels back to their source was fundamental to an 

 understanding of the lymphatic system, and I began with the group 

 in the neck as it was the primary group. The vessels in the neck 

 converge to a sac which is readily demonstrated by injection as is 

 shown in Fig. 1, Vol. IV, American Journal of Anatomy. This 

 sac, which lies against the internal jugular vein, is the beginning or 

 anlage of the lymphatic system. In embryo pigs from 14.5 to 16 

 cm. long there are symmetrical jugular sacs opening into the vein. 

 Saxer made mention of these sacs as a part of the lymphatic system, 

 but did not realize their fundamental significance.^^ These sacs are 

 either empty or contain a few blood corpuscles. Y. T. Lewis worked 

 on the stages before this lymphatic sac is formed and carried our 

 knowledge a step farther by showing that they are preceded by a 

 plexus of veins opening into the jugTilar vein.-^^ This plexns of veins 

 gradually becomes cut off from the main vein and by the coalescence 

 of the small veins a sac is formed which is entirely free from the 

 jugular vein for a time. Subsequently the symmetrical sacs rejoin 

 the veins. The endothelial lining of these sacs is thus derived from 

 the endothelium of the veins. In studying the lymphatics Dr. Lewis 

 used the method of graphic reconstruction. The fact that the jugu- 

 lar sacs are transformed venous capillaries, I was able to entirely 

 confirm by the method of injection in pig embryos.^- In pig embryos 



"Anat. Hefte, Vol. VI, 1896. 



"Lewis. The Development of the Lymphatic System in Rabbits. Amer. 

 Jour. Anat, Vol. V, 1906. 



"Sabin. Anat. Record, Vol. II, 1908. 



