14 Florence R. Sabin. 



on the lymphatics and the fact that they never join with the con- 

 nective tissue cells. 



Thus the observation of the growth of capillaries hy sprouting 

 was made over and over again where there was the chance of seeing 

 the process in the living form or of obtaining injected specimens, 

 and yet during all these years the observations were not generally 

 accepted, since as the fundamental morphology of the lymphatic sys- 

 tem was unknown, the significance of the observations could not be 

 understood. 



It is, however, not true that every observer who studied the living 

 tadpole's tail described the process of the sprouting of the endothelial 

 wall of blood vessels and of lymphatics as showing the method of 

 growth. Indeed at least three observers have doubted whether there 

 are two different kinds of capillaries in the tadpole's tail. I have 

 already noted that Strieker doubted the presence of lymphatic capil- 

 laries in the tadpole's tail. 



Wysotzky (155), whose work, since it is published in Eussian, 

 I quote from Mayer (89), thought that the capillaries without blood 

 were merely young blood capillaries. 



Sigmund Mayer (89), in an interesting and valuable paper, " Ueber 

 die Blutleerengefasse im Schwanze der Batrachierlarven," gives first 

 an excellent analysis of the literature on the subject of the capillaries 

 in the tadpole's tail. He brings out the fact that it was the presence 

 of an occasional red corpuscle in the "lymphatic" which was the 

 stumbling block in accepting the presence of lymphatic capillaries. 

 He then records an observation which in reality cleared up the entire 

 difficulty in regard to the " empty capillaries." As E. R. Clark (26) 

 has shown, Mayer, however, failed to see the bearing of his own 

 observations. Mayer used curare and the electric current to anaes- 

 thetize the larva and then covered the tail with a cover slip. When he 

 sucked the water out from under the cover slip he noted that the 

 blood would gradually stop flowing or even go in the reverse direction 

 in the blood capillaries and that the vessel would collapse, so that it 

 looked like a solid cord. Thus he also observed the contraction of 

 endothelium. Furthermore, he noted that the " other empty vessels " 

 (namely, lymphatics), contracted more frequently than the blood 

 capillaries. When, however, he studied these other empty vessels he 

 thought that the occasional cells in them flowed now toward the center, 

 now toward the periphery, and he ended with the somewhat vague 



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