156 ADAM M. MILLER 



the chain of lymphatics with which the blood cells have for the 

 most part been associated. 



Chick embryo of nine days and fourteen hours {Columbia Collec- 

 tion, series no. 320). Reconstruction, ventral view and viewfrorn left 

 side. Figures 27 and 28. At this stage the main features of the 

 adult thoracic duct have been established. The large plexus {17) 

 ventral to the aorta {1) now drains through the right and left 

 thoracic ducts {17a) into the 'approaches' {15a) of the jugular 

 lymph sacs {15) and then through the sacs into the great veins 

 {10, 12). 



The ventral plexus {17), the isolated anlagen of which were 

 the first lympathics to develop in the thoracic duct line, is rela- 

 tively smaller and the component channels less dilated than in 

 the preceding stages. This is probably due to the outflow of the 

 contents into the lymph sacs and veins through the more recently 

 formed channels which have been called the right and left tho- 

 racic ducts {1 7a) . The mesenteric lymphatics {21 ) have increased 

 in size and for the most part coalesced to form sinus-like chan- 

 nels. Between these and the thoracic duct chain no connection 

 can be detected at this stage, although little tissue intervenes. 

 Farther cephalad another isolated group of lymphatics {22) is 

 associated with the esophagus. 



The right and left thoracic ducts {17a) are longer than in the 

 preceding stage (cf. fig. 26, 17a). The left is considerably greater 

 in diameter than the right. Each opens into the corresponding 

 'approach' {15a) of the jugular lymph sac. The 'approach' on 

 each side is still patent as a branch of the main portion of the sac. 



There is now free communication between the lymph sacs 

 {15) and the great veins through recently formed taps, of which 

 two are present on the left side of the embryo and one on the 

 right. Of the two on the left side, one is situated on the dorso- 

 mesial side of the superior vena cava {12) just below the level of 

 the jugulo-subclavian junction {10, 11); the other is situated 

 farther forward on the mesial side of the jugular vein {10). The 

 tap on the right side of the embryo is located on the dorso-mesial 

 side of the superior vena cava at the level of the jugulo-subclavian 

 junction. 



