THE DEVELOPMENT OF THE VASCULAR SYSTEM 279 



openings which represent the lymphatico-venous communications of the 

 adult (Fig. 253, d). In the case of the posterior sac, intercellular spaces 

 dorsal to the posterior cardinal vein (.Fig. 254, 8) first form a plexus the 

 components of which then unite into a large endothelial-lined space which 

 opens into the dorsal tributaries of the vein. 



The thoracic duct also arises as a chain of isolated endothelial-lined 

 spaces along the line of the aorta. These unite longitudinally into a con- 

 tinuous channel which joins the jugular lymph sac, thus forming the axial 

 lymphatic drainage line of the body (Fig. 255, a, b, c). In reptilian embryos 

 the spaces first fuse into a distinct periaortic plexus out of which the thoracic 

 duct is established. In the avian embryo the chain of spaces follows the 

 general line of the aorta but does not become so intimately associated with 

 the great arterial trunk as in reptiles. In the mammalian forms rudiments 

 of the thoracic duct follow the same general plan of development, but are 

 associated topographically with the ventro-medial tributaries of the azygos 

 veins. These tributaries finally become detached from the larger venous 

 trunks, atrophy and disappear, being replaced by the thoracic duct. 



On the same principles laid down for the development of the lymph sacs 

 and thoracic duct, the peripheral lymphatics also are developed. In all 

 the regions of the body not immediately drained by the lymph sacs or 

 thoracic duct, mesenchymal intercellular spaces enlarge and coalesce, the 

 cells bounding the coalesced spaces being transformed directly into endo- 

 thelium; the spaces unite to form a plexus of endothelial-lined channels, 

 and in this plexus certain channels increase in size to form the larger lym- 

 phatics which converge and eventually join the main axial drainage line. 

 Thus the lymphatic drainage of the entire body is established. 



One of the most interesting and significant phases of lymphatic develop- 

 ment, which has been brought out through recent studies of the problem, is 

 the role played by certain early lymph channels in conveying blood cells to 

 the general circulation. It has been found that, in the region subsequently 

 occupied by the lymph sacs, extensive blood cell formation (haemopoiesis) 

 occurs prior to the formation of lymphatic rudiments. As the lymph spaces 

 appear and unite into a plexus the developing blood cells are included within 

 them (Fig. 253, a and Fig. 254, 10). When the lymphatic plexus joins 

 the veins the blood cells are carried into the general circulation (Fig. 253, b). 

 This haemophoric function of the early lymph channels is especially prominent 

 in the case of the thoracic duct in the chick. Here extensive collections of 

 blood cells develop in the mesenchymal tissue along the line of the aorta 

 and become included within the rudiments of the thoracic duct and eventu- 

 ally, when the latter unites with the jugular lymph sac, are carried into the 

 veins and thus enter the general circulation. After these early lymphatics, 



