The Lymphatic System in Human Embryos. 79 



these veins, or does the duct grow from the two sacs, the cisterna 

 chyli and the jugular one. For this second view the evidence is 

 also weak, it consists in this, that other lymph ducts wherever we 

 can study them grow from the sacs ; and secondly in pig embryos and 

 in human embryos one can trace a duct fonvard from the cisterna 

 chyli and caudal ward from the jugular sac, and in later stages these 

 two ducts have joined. The weakness of this evidence lies in the 

 fact that in the earlier stages the picture is always liable to be con- 

 fused by Lewis' multiple anlagen. In both pig and human embryos 

 the stages to be studied for the thoracic duct lie between 20 and 30 

 mm. In an embryo pig the complete thoracic duct can be injected at 

 27 mm. It should be quite clearly noted that whichever method 

 of formation of the thoracic duct proves ultimately to be correct, 

 that is whether it arises from the azygos veins in situ or from an out- 

 growth of the lymphatic sacs, the most fundamental point remains 

 the same, that its endothelium is vascular. 



However, it should be stated here that wherever growling lym- 

 phatic capillaries have been absolutely tested, they grow by the 

 sprouting of their own endothelium rather than by additions of new 

 anlagen from the veins. This has already been noted for the living 

 lymphatics; it is also shown by the work of Dr. H. M. Evans^^ 

 on the growth of new lymphatic capillaries into a sarcoma of the 

 intestine. His injections show that the new lymphatic capillaries 

 are derived from the mucosal plexus and that these new vessels are 

 analogous with the central lacteals of the villi. In the tumor, how- 

 ever, they are developing beyond the normal limits of the tenninal 

 lacteals into a spreading plexus. This plexus shows all gradations 

 as seen in Evans' figure to the normal lacteal. 



From the next specimen (No. 86), measuring 30 mm., a graphic 

 reconstruction has been made of the entire primitive lymphatic sys- 

 tem. Fig. 12. It does not show the extent of the peripheral lym- 

 phatics, but does show the relations of the primitive system. At 

 this stage, as will be seen in the reconstruction, the lymphatic system 



"Evans. On the occurrence of newly-formed lymphatic vessels in malignant 

 growths. Johns Hopkins Hospital Bulletin, 1908. 



