en* /.'. 8abin. 



_.•■ (Sabin 130), which are easy to identify. The method 



-. however, inadequate t" define the Limits of growth, 



the large vessels, for the tiny lymphatic Bproute can only 



found in sections when the plane of cutting is favorable. 



This pi d of the lymphatics in the Bkin has been confirmed 



by Polinski (107) in bovine embryos. The patterns which he Bhows 



make a most interi rison with those of pig, cat and human 



embryos. In the bird the same progression of zones is shown to 6ome 



hi in the injections of Mierzejewski (96). 



Beside the definite zones the lymphatics grow into definite layers. 

 !,' aviei (118) ooted thai the superficial lymphal ics are deeper than the 

 blood vi --• -. They follow the vessels into the skin and each plexus - 

 deeper than the corresponding blood vascular plexus (Sabin 130). The 

 development of the lymphatics into the intestine, as worked out by 

 Beuer i 13), illustrates the point of the definite layers into which the 

 lymphatics grow particularly well. The lymphatics for the intestine 



iw from the retroperitoneal Bac, which arises from the renal veins 

 (Baetjer, 8). The vessels form a rich plexus in the mesentery and 

 from this plexus a series of vessels grow into the intestinal wall and 

 penetrate to the submucosa. II< re they form at first a of units 



shown by Heuer in fig. I" (43), which has been copied as fig. 508 

 ( Sabin 134 ). These primary vessels soon form the submucosal plexus 

 of ducts, from which the mucosal capillary plexus and lacteals develop 

 on the one hand and the serosal vessels on the other. 



This fact, thai the body is invaded by lymphatics, bas an especial 

 interest, because il was the clue which enabled us to trace back the 

 lymphatic ducts to their beginnings and show that they 

 arise from the veins and grow toward the periphery. It is, further- 

 more, the key h\ which the developmenl of the lymphatic system 

 within each organ may be traced to the ultimate capillaries. [1 is 

 considered again in relation to the development of lymph sacs, for 

 re it can l>c taken up in connection with its exact relations to 

 vaseular and non-vascular zones. Both the blood vascular and the 

 lymphatic systems invade the body by the growth of their capillaries. 

 I systi 'M has it- definite zones and layers which can be followed in 

 orderly progression. Tee invasion of the body by the blood capillaries 



• i plete than by the lymphatic capillaries; Eor example, the 



lymphatic capillaries do nol grow into the centra] nervous system, nor 

 into the int< rspaces between musi le fibers. 



