22 Florence R. Sabin. 



were filled with blood and that the blood in them was stagnant, in 

 striking contrast to the rapidly circulating venous blood. The fact 

 that the lymphatics invade the skin in the chick while they are in the 

 blood-filled stage gives at once an advantage. By means of fine glass 

 cannulas E. L. Clark was able to inject individual vessels of this blood- 

 filled plexus in the living chick and watch the ink flow to the pul- 

 sating lymph heart and thence to the veins. These blood-filled vessels 

 have the usual characteristics of lymphatic capillaries, in that they 

 are larger and more irregular than blood capillaries, but the essential 

 point in E. L. Clark's work is that by using a form in which the lym- 

 phatics could be seen in the living embryos she discovered a new 

 criterion for recognizing lymphatics, namely, the stagnant blood in 

 contrast to the circulating blood. 



With this important criterion she has followed back the origin of 

 the posterior lymph hearts. Since Sala (137) it has been known that 

 in the chick the posterior lymph hearts develop opposite the lateral 

 branches of the first five cocc3 T geal veins (137, p. 269). Sala described 

 the process as beginning during the, sixth day by a hollowing out of 

 spaces in the mesenchyme, but by watching the living chick embryo 

 during the fifth day E. R. and E. L. Clark have seen that the skin 

 over the first five coccygeal veins is a comparatively non-vascular zone, 

 so that brilliant direct illumination enables them to see in the depth 

 a series of tiny blood-filled buds close to the main coccygeal vein and 

 its branches. The blood in these buds is of slightly different color 

 from the circulating blood. They proved that these buds are always in 

 connection with the parent vein, for a direct injection of them always 

 runs over into the vein, but no injection of the peripheral blood capil- 

 laries ever fills them. Hence they bud off from the veins and are filled 

 with blood from a back-flow from the parent vein. It is of course evi- 

 dent that this process cannot be seen in the body wall of the chick with 

 the clearness with which every cell division can be followed in the 

 tadpole's tail; nevertheless, the stagnant blood has been proved by injec- 

 tion to practically fill the developing vessels, so that it is an adequate 

 criterion of the extent of their development. 



By selecting a chick which shows these primary lymphatic buds and 

 keeping it under observation under high power in a warm chamber 

 E. E. and E. L. Clark (29) have been able to watch these blood-filled 

 lymphatic buds join with each other to form a deep circumscribed 

 lymphatic plexus, the anlage of the posterior lymph heart. It is clear 



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