The Lymphatic System in Human Embryos. 77 



interesting relations of the cisterna chyli. It arises opposite the 

 second, third and fourth lumbar vertebrae, closely adjacent to the 

 inferior vena cava where it anastomoses with the azygos veins. In 

 studying through Dr. Mall's collection I have become convinced that 

 the cisterna chyli forms one of the primitive sacs and that the thoracic 

 duct may grow forward, i. e., anteriorly from it. Baetjer has shown 

 •that the mesenteric sac soon becomes connected with the cisterna 

 chyli. In this series I cannot find any evidence of a thoracic duct 

 The cisterna chyli differs from the other sacs simply in not being 

 completely transformed into lymph nodes, though its lower border 

 develops into a large group of them, as will be shown in the last 

 series. 



The next embryo of the series (No. 6), measuring 24 mm., has a 

 large jugular sac. The series is incomplete so that I cannot get the 

 length of the sac, but its width is the same as the preceding, namely 

 1 mm. The valves are present and the sac shows much bridging. 

 There is a well defined subclavian sac. This series is also more in- 

 teresting in connection with the other sacs. It shows three things, 

 the retroperitoneal sac, the cisterna chyli, the beginning thoracic 

 duct and the anlage of the posterior lymph sacs. A point of especial 

 interest in this series is in connection with the cisterna chyli. This 

 sac is present as a few vessels dorsal to the aorta ; and from the sac 

 ducts extend anteriorly immediately adjacent to the azygos veins. 

 On the left side, this duct extends into the thoracic cavity almost 

 to the neck. I cannot trace it to the jugular sac nor is the series 

 perfect enough to enable one to say whether it is present in every 

 section or not, but there is sufficient evidence to indicate that the 

 thoracic duct may be an outgrowth of the cisterna chyli. 



The thoracic duct has proved to be the most difficult part of the 

 lymphatic system to work out for this reason, we have not yet found 

 a way to inject it in early stages and uninjected sections are not 

 adequate. The evidence of sections is as follows, the jugular sac 

 and the cisterna chyli, which the duct subsequently connects, develop 

 before the duct. The question is, does the duct develop from 

 multiple anlagen from the azygos veins for which there is no proof 

 except that lymphatic vessels can be seen in sections adjacent to 



