August 4, 1916] 



SCIENCE 



157 



together after cell-division forming strands, 

 and secondly, a tendency to join other 

 bands of similar cells by protoplasmic proc- 

 esses. These bands of cells become blood 

 vessels. 



It is, I think, clear that the question now 

 to be solved is how long does endothelium 

 continue to differentiate out of mesen- 

 chyme? It can be seen to differentiate in 

 the living chick in all the stages I have yet 

 studied, that is in the stages before the cir- 

 culation is established. This covers ap- 

 proximately the first two days of incuba- 

 tion. As is well known, there is a group of 

 anatomists — Maximow, Reichert and Mol- 

 lier, and a group of American workers, 

 notably Huntington and McClure, who be- 

 lieve that endothelium continues to differ- 

 entiate out of mesenchyme possibly 

 throughout life. From the evidence which 

 I have previously given I think it much 

 more likely that endothelium will prove to 

 have a limited period of differentiation, 

 followed by growth. The study of the 

 origin of blood-vessels seems to me to em- 

 phasize again the endothelial cell and to 

 show that the vascular system arises from 

 a differentiation, and growth of endothelial 

 cells rather than by a dilatation of spaces. 



In looking back over the history of the 

 development of our knowledge of the lym- 

 phatic system, it is very clear that there 

 have been periods of great activity fol- 

 lowed by periods of rest. We are at pres- 

 ent in a period of activity, and I should 

 like to sum up what seem to me to be the 

 results of the work of the last fifteen 

 years. It has been shown that the problem 

 of the origin of the lymphatic system is but 

 a part of the general problem of the origin 

 of the vascular system. Lymphatics are 

 modified veins, in the sense that they grow 

 from the veins. The veins are the primary 

 absorbents and continue to take part in ab- 

 sorption throughout life. Up to the time 



of about six weeks for the human embryo, 

 they are the only absorbents. Subsequently 

 other systems develop, such as the arach- 

 noidal villi and the lymphatic vessels, to 

 assist in the function of absorption. The 

 lymphatics only partially invade the body, 

 and present indications point to the fact 

 that their functions in absorption may be 

 to some extent specific. 



In an injection into the tissues of a dead 

 organism it is essential to puncture the ves- 

 sels of a plexus of lymphatic capillaries in 

 order to fill lymphatics with a non-diffus- 

 ible fluid. These injections demonstrate a 

 complete wall, in the anatomical sense, 

 which is ruptured only by increased pres- 

 sure. In the living animal both true solu- 

 tions and granules pass into lymphatic 

 capillaries through the activities of endo- 

 thelial cells or by means of wandering 

 phagocytic cells. 



This conception of the lymphatic system 

 is at variance with the older idea of hazy 

 lymphatic capillaries that faded off indefi- 

 nitely through hypothetical lymph radicals 

 into the tissue spaces. With the newer con- 

 ception of definite lymphatic capillaries of 

 endothelium it would be much better if we 

 should revise the terms which developed in 

 the period when our theories were vague 

 and indefinite. In the first place there 

 are "blood-capillaries," "lymphatic cap- 

 illaries" and "tissue-spaces." If we 

 should reserve the term "plasma" for the 

 fluid within the blood-vessels, "lymph" for 

 the fluid within the lymphatics and 

 "tissue-fluid" for the fluid within the 

 tissue-spaces, it would be a great gain in 

 clearness. The term "tissue-fluid," mean- 

 ing the fluid which is in the tissue-spaces 

 of the living animal, should not be con- 

 fused with the term "tissue- juice," by 

 which the physiologist means the fluid 

 which can be pressed out of the tissues. 

 The term tissue-fluid should include such 



