THE DEVELOPMENT OF THE VASCULAR SYSTEM 



275 



maintain that the lymphatics and their endothelium arise in situ from 

 intercellular tissue spaces and the bordering cells, argue that the same 

 principles underlie the formation of lymphatics that determine blood-vessel 

 development and that it has been shown experimentally that blood vessels 

 develop in regions which have been entirely cut off from any source of endo- 

 thelium except the mesenchymal cells 

 in situ (see page 225). 



According to the first view lym- 

 phatic development can be divided into 

 two stages: (i) the formation of isolated 

 lymph sacs, derived from veins, which 

 become united into a system, and (2) 

 the peripheral growth of lymph vessels 

 which sprout from the endothelium of 

 these sacs and spread through the body, 

 (i) The first sacs appear, one on each 

 side, along the jugular (anterior cardi- 

 nal) veins. The branches of these veins 

 at first form a plexus; a portion of the 

 plexus becomes cut off from the parent 

 stems and lies as a series of isolated 

 spaces in the mesenchyme ; these spaces 

 then enlarge and coalesce to form an en- 

 do thelial-lined sac the jugular lymph 

 sac or heart which afterward joins the 

 jugular vein by a new opening (Fig. 

 251). A second pair of sacs the pos- 

 terior lymph sacs or hearts develops 

 in the same manner from the more 

 caudal branches of the posterior cardi- 

 nal veins (Fig. 251). Two other sac- 

 like structures develop the cisterna 



chyli and retroperitoneal sac the former in the region of the renal veins 

 and the latter in the vicinity of the suprarenal bodies. Through the longi- 

 tudinal fusion of the chain of sac-like structures, the axial lymphatic drain- 

 age line of the body is established (Fig. 251). The thoracic duct probably 

 represents the fused cisterna chyli and jugular lymph hearts. The lymph 

 hearts in the avian and mammalian embryo become relatively smaller as 

 development proceeds until in the adult they are barely discernible as slight 

 dilatations in the lymph vessels. The cisterna chyli, however, may persist 

 as a clearly distinguishable dilatation at the caudal end of the thoracic duct. 



FIG. 252. Diagram showing network of 

 lymphatic vessels in skin of pig embryos. 

 Sabin. 



Area marked A shows extent of network in 

 an embryo of 18 mm.; B, in embryo cf 

 20 mm.; C, in embryo of 30 mm.; D, in 

 embryo of 40 mm. 



