20 Florence B. Sabin. 



capillaries, fig. 6 (Sabin 130), which are easy to identify. The method 

 of sections is, however, inadequate to define the limits of growth, 

 except for the large vessels, for the tiny lymphatic sprouts can only 

 be found in sections when the plane of cutting is favorable. 



This progression of the lymphatics in the skin has been confirmed 

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

 make a most interesting comparison with those of pig, cat and human 

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

 extent in the injections of Mierzejewski (96). 



Beside the definite zones the lymphatics grow into definite layers. 

 Eanvier (118) noted that the superficial lymphatics are deeper than the 

 blood vessels. They follow the vessels into the skin and each plexus is 

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

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

 Heuer (43), illustrates the point of the definite layers into which the 

 lymphatics grow particularly well. The lymphatics for the intestine 

 grow from the retroperitoneal sac, 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. Here they form at first a series of units 

 shown by Heuer in fig. 10 (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, that the body is invaded by lymphatics, has an especial 

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

 systems of lymphatic ducts to their beginnings and show that they 

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

 more, the key by which the development of the lymphatic system 

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

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

 there it can be taken up in connection with its exact relations to 

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

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

 Each system has its definite zones and layers which can be followed in 

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

 is more complete than by the lymphatic capillaries; for example, the 

 lymphatic capillaries do not grow into the central nervous system, nor 

 into the interspaces between muscle fibers. 



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