16 Florence B. Sabin. 



If it be established that the peripheral capillaries grow entirely 

 from preceding capillaries, then it is a perfectly logical hypothesis 

 that the lymphatics grow from the veins, from center to periphery, 

 rather than from the periphery to the veins. This hypothesis was 

 advanced by Eanvier (113), based on the evidence of the method of 

 growth of the lymphatic capillaries within the peripheral plexus. 

 Eanvier (110-118), in a series of studies between the years 1895 and 

 1897, made' injections of lymphatics in a variety of embryos. For 

 example, he injected the exceedingly abundant plexus in the mesentery 

 of a foetal pig 10 cm. long, as well as in the rat and rabbit. He also 

 injected the lymphatics in the amphibia. These injections gave him an 

 extensive knowledge of the form of the developing lymphatic plexuses, 

 the valves looking like collarettes in the walls of lymphatic ducts, the 

 irregular capillaries with blunt terminal knobs, of which especially 

 large ones are often found in the mesentery as well as the slender 

 sprouts, either ending free or as bridging a gap in the network to 

 another vessel. From all of these forms of growing lymphatics Ean- 

 vier saw that the growth of lymphatics was from center to periphery, 

 and even though he thought that the lymphatics did not begin in a 

 mammalian embryo until it was about 10 cm. long, a stage at which the 

 embryology of the system may be called complete, nevertheless he went 

 a step farther than all of the preceding observers, for he deduced the 

 hypothesis that the lymphatics grow from the veins. He likened their 

 growth to the growth of a gland (113). In my own work I was led 

 to the same view by tracing the lymphatics in younger and younger 

 embryos back to the veins, and the next section will include the evi- 

 dence which I regard as essential to establish the hypothesis of Eanvier 

 as the correct theory of the origin of the lymphatic system. The ac^ 

 count of the evidence for growth by sprouting is, however, not yet 

 complete. As far as the evidence can be obtained by the observation 

 of injected specimens, it was especially well described and figured 

 by MacCallum (79). He added the observation of watching the in- 

 jection under the microscope and showed that extravasations are due 

 to explosive rupture of the lymphatic wall. 



Injected specimens of developing lymphatic capillaries are now 

 well known; excellent figures are given by Battels (10, figs. 9 and 10, 

 p. 43). They show a large number of sprouts, because they are taken 

 from the skin of a pig embryo 6.5 cm. long. This is the exact stage 

 when the secondary, superficial, fine-meshed plexus is just growing 



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