The Lymphatic System in Human Embryos, 51 



tery of a pig embryo 17 to 19 mm. long. It is readily noted that 

 these small veins are injected from the main veins as the drawings 

 show the injected ink of the specimens. As the embryo develops, these 

 small veins enlarge and coalesce to form a sac, which shows a few 

 connections with the veins, as proved by injection until the embryos 

 are 23 mm. long. The sac is completely formed at 30 mm., when 

 it is cut off from the veins entirely and clearly connected with the 

 cisterna chyli. Baetjer's series of nine drawings show every stage 

 in process of the transformation of the veins into the sac and its 

 subsequent connection with the lymphatic system. 



Thus to sum up, it will be seen that the lymphatic system begins as 

 a series of sacs of which eight haye been described ; three paired, the 

 jugular sacs, the subclavian and the posterior lymph sacs; and two 

 unpaired, the retroperitoneal and the cisterna chyli. In the human 

 embryo there are only six, for the subclavian sacs are extensions of 

 the jugular sacs. All of the sacs are shown in Fig. 12, in a human 

 embryo 30 mm. long. The method of origin of two of them, namely 

 the jugular sacs and the retroperitoneal sacs, has been worked out 

 with care showing that they are clearly derived from the veins. The 

 jugular sacs form a secondary connection with the jugular veins, the 

 other sacs forming in regions where there is great shifting of veins 

 do not form secondary communications with their own veins but 

 join the other lymph sacs to make a primitive system. 



The question now arises whether these sacs can be considered as 

 analogous with the amphibian lymph hearts. None of the mamma- 

 lian sacs studied develop any muscle in their walls ; throughout their 

 history they have a lining simply of endothelium, but they all are in 

 regions from which ducts radiate out to drain wide areas, so that as 

 the system begins to function the lymphatic stream converges to 

 these sacs and in this sense they represent the lymph hearts. In the 

 chick the posterior lymph sacs are true lymph hearts, for they develop 

 a muscular wall, and from Sala's description it is easy to see that 

 these hearts really arise by exactly the same process as the mamma- 

 lian sacs. The fate of the lymph sacs has some bearing on the 

 subject. ^^ This has been followed for the jugular sac in the pig and 



"Sabin. Amer. Jour. Anat, Vol. IV, 1905. 



