THE DEVELOPMENT OF 1HE VASCULAR SYSTEM. 



247 



and into the inferior vena cava and renal veins (in some monkeys) , is second* 

 arily established. The same hydrodynamic mechanical factors regarded 

 as operative in the formation of blood vessels, viz.: pressure and friction 

 incident to blood flow (see page 195), are considered as effective likewise 

 in the development of lymphatics. Fundamentally, therefore, the lymph 

 vascular system from the viewpoint of development differs in no wise from 

 the blood vascular system. 



o-l 



FIG. 2 1 7. Diagrams showing three stages (a, b, c) in the development of the thoracic duct in 

 the cat embryo, in which the jugular lymph sac has established two permanent venous 

 connections (8 and 9). Huntington. 



i, Anterior cardinal vein; 2, duct of Cuvier; 3, posterior cardinal vein; 4, external jugular- 

 cephalic vein; 5, subclavian vein; 6, jugular lymph sac; 7, thoracic duct 'approach' of 

 lymph sac; 8, common jugular opening of lymph sac; 9, jugulo-subclavian opening of lymph 

 sac; 10, rudiments of thoracic duct; n, thoracic duct. 



The lymph sacs, both jugular and posterior, arise through enlargement 

 and confluence of mesenchymal intercellular spaces, the cells bounding the 

 spaces becoming flattened and rearranged to form endothelium. In the 

 case of the mammalian (cat) jugular sac, intercellular spaces in the region 

 dorso-lateral to the anterior cardinal vein unite into an intricate plexus of 

 channels which then opens into the vein (Fig. 215, a and b). Following 

 this the components of the plexus enlarge and coalesce to form the sac, 

 which then temporarily severs connection with the vein (Fig. 2 1 5, c) . Finally 

 the sac effects a permanent connection with the vein through one or more 



