The Origin and Development of the Lymphatic System. 19 



Subsequently this plexus develops valves, but at this stage the entire 

 lymphatic system can theoretically be injected from any one vessel, 

 because there are no valves, or, in other words, because it is entirely 

 a capillary system. Nearly the same stage is shown for a human 

 embryo in figs. 505 and 506 (Sabin 134). This was a remarkable spec- 

 imen, in which air filled the lymphatics, and it has the additional in- 

 terest of showing the beginning of valves in the vessels of the skin, 

 namely, those of an occipital duct and in the axillary and inguinal 

 ducts. 



If earlier stages of embryo pigs are taken the cutaneous vessels, as 

 shown by injecting them, gradually recede, as seen in figs. 1 to 4 

 (Sabin 130), to two points, one in the neck and one in the groin. 

 The earliest lymphatics reach the skin in the neck of an embryo pig 

 about 18 mm. long. All injections of the lymphatics of the skin of 

 the anterior part of the body between the stages of 2 and 5 cm. long 

 can be seen to run into two symmetrical sacs in the neck close to the 

 jugular vein. These anterior lymph sacs, which have now been identi- 

 fied in a very great number of forms, can be found in pig embryos in 

 earlier stages than the first cutaneous lymphatics. In my first paper 

 I traced them back to the stage of 14 mm., when they are small empty 

 sacs, close against the anterior cardinal vein. In reality they begin 

 in the pig of 10 to 11 mm. and their method of origin will be discussed 

 in the next section. 



The argument of lymphatic and non-lymphatic zones thus consists 

 in the fact that the first lymphatics are sacs close to the veins, and 

 from this stage onward lymphatics can be injected in wider and wider 

 zones until they cover the entire surface of the body. 



It is necessary to prove that each zone represents the limit of the 

 growth of the lymphatics for a given stage. To prove an injection is 

 complete for any stage such as one shown in fig. 6 (Sabin 135), it is 

 essential to have abundant material and to show by injection that the 

 lymphatic tips always rupture along a given line when the pressure 

 is increased. The shape of the lymphatic vessels at the line of growth 

 will vary according as the vessels are progressing rapidly or not; for 

 example, the rounded ends of fig. 2 (Sabin 129) are from an area 

 where lymphatics grow slowly, while the margins of fig. 6 (Sabin 135), 

 with numerous pointed sprouts, are from a rapidly growing zone. Fur- 

 thermore, sections of the skin in the lymphatic and non-lymphatic zones 

 show a sharp contrast in the presence or absence of the large lymphatic 



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