18 Florence R. Sabin. 



Thus there can be no question in regard to the method of growth 

 of the peripheral capillaries. Both blood and lymphatic capillaries 

 grow by the activity of the protoplasm of their walls. The endo- 

 thelium at the growing tip is a sj'ncytium, the processes advance and 

 recede; they are tiny, hair-like vessels, and they receive their nuclei 

 from the parent stem. There are differences in the appearance of the 

 two types of capillaries probably associated with the presence and 

 absence of the circulating blood and the difference of function. The 

 lymphatics have very many more tiny sprouts, only a few of which 

 are permanent; that is, their wall is much more amoeboid and the 

 vessels much more irregular. Thus on positive evidence, namely, the 

 observation of the process in the living form, which is the best evi- 

 dence known to anatomical science, we are justified in the conclusion 

 that capillaries grow by sprouting. 



B. LYMPHATIC AND NON-LYMPHATIC ZONES. 



The second argument in favor of the venous origin and the centrif- 

 ugal growth of lymphatics by their own endothelium is that it can 

 be shown that lymphatic vessels gradually invade the body from certain 

 limited centers. Thus if injections of vessels are made from these 

 centers in larger and larger embryos an ever-increasing zone of 

 lymphatic capillaries can be demonstrated. Moreover, for each stage 

 the zone of vessels is constant. 



I began my work with the study of the development of lymphatic 

 glands in the embryo pig and to obtain injections of them introduced 

 the needle into the foot pads. By taking younger embryos a stage 

 was soon reached when no injections of the foot pads ever entered 

 lymphatics, but at these stages one could still inject lymphatics in the 

 skin over the body. This was the beginning of the proof that lym- 

 phatics bud off from the veins and grow peripheralward instead of the 

 reverse (Sabin 129). 



In an embryo pig 5 cm. long (Sabin 130, fig. 5) a simple plexus of 

 lymphatic capillaries has almost entirely covered the body. I say capil- 

 laries advisedly, adopting Ranvier's (118, p. 74) criterion of valves 

 for lymphatic ducts in contrast to their absence in the capillaries. 

 The body is first invaded by lymphatic capillaries, and the primary 

 plexus shown in fig. 5 (Sabin 130), which is the anlage of the deep, 

 subcutaneous plexus, was injected by a single puncture of the needle. 



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