58 Florence R. Sabin. 



experiment. He made a careful camera liicida drawing of the lym- 

 phatics in the living tadpole's tail, then killed the tadpole and cut it 

 in serial sections and tried to reconstruct the lymphatics. The 

 failure in reconstructing these amphibian lymphatics confirms similar 

 attempts of my own on mammalian lymphatics and makes me feel 

 sure that uninjected capillaries cannot be completely reconstructed. 



The same point in connection with the blood vascular system, 

 namely that the blood capillaries cannot be reconstructed from unin- 

 jected specimens no matter how perfect, will be conceded, but has 

 been brought out much more strikingly by the work of Dr. Evans, 

 soon to be published from this laboratory, for he has shown that a 

 blood capillary plexus can be demonstrated by injection where it 

 was not known to exist before. 



Thus the question of the relation of the peripheral vessels to the 

 sacs is becoming more and more clear. There is a primary 

 lymphatic system which consists of sacs that are formed directly 

 from the veins. These primary lymphatic sacs are transformed 

 from a series of isolated sacs into a system by means of the thoracic 

 duct and the right lymphatic duct. These two structures form a part 

 of the primary system. The secondary system consists of the 

 peripheral vessels which, it is becoming more and more sure, are an 

 outgrowth from the sacs. Thus we can say that the primary system, 

 as far as it is made up of sacs, comes from transformed veins, and 

 that the secondary system, characterized by being formed of lymphatic 

 ducts and capillaries, develops by endothelial sprouting from the 

 sacs. It remains now to be determined whether the thoracic duct 

 develops after the manner of the primary sacs as transformed branches 

 of the azygos veins or whether it develops as the other lymphatic 

 ducts of the body do, from endothelial sprouts from the sacs. No 

 theorizing can decide between these two ideas. We must wait some 

 decisive method of getting at the facts. The presumption seems 

 to us to lie on the side that the thoracic duct develops in the same 

 manner as all the other ducts, since wherever the isolated anlages of 

 ducts can be tested, as, for example, in the living tadpole's tail, they 

 prove not to exist. Moreover, Dr. McClure, who is at present the 

 advocate of the idea that the thoracic duct arises as a series of 



