August 4, 1916] 



SCIENCE 



153 



grow by the multiplication of endothelial 

 cells. The opposing theory holds that the 

 lymphatics arise from tissue-spaces and 

 grow by adding on new tissue-spaces; that 

 beyond the tip of a definite completed ves- 

 sel, which can be injected, are tissue-spaces 

 which will be added to the capillary. 



It is here necessary to submit the differ- 

 ent types of method and the nature of the 

 evidence which has been brought forth 

 under the stimulus of these two theories. 

 Some of the methods are direct, some in- 

 direct, but in all there is an effort to under- 

 stand the nature of that very interesting 

 and important tissue, the endothelial cell. 



First, in regard to the nature of the 

 earliest lymphatic buds, it is clear from 

 sections, both of mammals and of birds, that 

 these buds are lined by endothelium, but it 

 proved very difficult to determine from sec- 

 tions that these buds were from the begin- 

 ning connected with the veins. Eleanor 

 Clark, 14 however, was able to test this point 

 in the case of the lymphatics of the chick by 

 developing a method for observing the tiny 

 red buds in the living embryo. Into these 

 lymphatic buds she injected a few granules 

 of ink, and then observed the granules en- 

 tering the vein. Moreover, in the amphibia 

 Fedorowicz 15 has traced each step of the 

 origin of the lymphatic buds from the veins, 

 by specific differences between the endo- 

 thelium and the mesenchyme. 



From these early lymphatic buds it is pos- 

 sible to inject an increasing plexus of lym- 

 phatic capillaries as the embryo develops, 

 and by this method to follow the lymphatic 

 capillaries to their form in the adult, in the 

 few places where that form is known. On 

 this evidence was based the theory of the 

 centrifugal invasion of the body by lym- 



« Clark, E. B. and E. L., Anat. Record, Vol. 6, 

 1912. 



is Fedorowicz, S., Bull. d. I' Acad. d. Sciences d. 

 Cracovie, 1913. 



phatic capillaries. The next method of 

 study which occupied the attention of the 

 group of anatomists who were trying to fol- 

 low the development of the lymphatic sys- 

 tem was a comparison of the adequacy of 

 the method in injection with the adequacy 

 of the method of reconstruction of lym- 

 phatics from serial sections as applied to 

 the problem of growth. This long series of 

 studies followed an observation of Lewis 16 

 that if the lymphatics were reconstructed 

 from sections they would appear as isolated 

 vesicles for which no connections could be 

 found. This is the experience of all who 

 attempt to reconstruct an uninjected capil- 

 lary plexus from sections, and therefore it 

 has been necessary to test the limitations of 

 the method. It is claimed that the method 

 of reconstruction reveals more lymphatics 

 than can be shown by the injection method, 

 as it shows not only all the lymphatics which 

 can be injected, but also the spaces that will 

 be added to the plexus later. Moreover, it 

 is on the evidence of reconstructions that 

 the theory of the growth of lymphatics by 

 the addition of tissue-spaces is based. It is 

 true, of course, that injections would not fill 

 up solid sprouts of endothelium, and every- 

 one who has made injections of lymphatics 

 is familiar with the difficulties of obtaining 

 perfect specimens, but it has been demon- 

 strated that when an area is chosen which 

 can be adequately injected, more of a capil- 

 lary plexus can be shown than can be re- 

 constructed. For example, Eleanor Clark 17 

 has published a picture of an injection of 

 the jugular lymphatic plexus of a chick 

 which showed a far more extensive plexus 

 than was demonstrated in a reconstruction 

 of the same stage, previously recorded by 

 Miller. 18 The two pictures, side by side, 



io Lewis, E. T., Amer. Jour, of Anat., "Vol. 5, 

 1906. 



it Clark, Eleanor L., Anat. Record, Vol. 6, 1912. 



is Miller, A. M., Amer. Jour, of Anat., Vol. 12, 

 1912. 



