GROWING LYMPHATICS AND THE MESENCHYME 379 



4, 5, 6 and 7). It might be said that the mesenchyme cells keep 

 the lymphatic at their finger tips, or that the lymphatic avoids 

 the mesenchyme cells. I have seen a mesenchyme cell approach, 

 in its wandering, a lymphatic lying across its course. Processes 

 were sent out by the mesenchyme cell on both sides of the lymphatic 

 so that the lymphatic lay within the U. In this case the processes 

 of the mesenchyme cell remained away from the wall of the lym- 

 phatic, and the cell moved past on one side, gradually withdraw- 

 ing the other arm of the U. 



In summing up the results of these studies, there are certain 

 facts on which I desire to lay especial emphasis. It will be 

 recalled that the purpose of the investigation was to obtain a 

 complete history of the growing lymphatic sprout and of the 

 growing mesenchyme cell individually and side by side, in order to 

 determine their relationship to one another — to find out whether 

 the growth of the lymphatic is brought about by the addition 

 of mesenchyme cells or of spaces lined by transformed mesenchyme 

 cells, or whether the growth of the lymphatic is independent of 

 the mesenchyme cells. The observations have furnished answers 

 to these questions which are perfectly clear. Each of the two 

 tissues has a characteristic independent life. The mesenchyme 

 cell wanders and increases by mitotic division. It maintains 

 throughout its indentity as a mesenchyme cell, and is not trans- 

 formed into lymphatic endothelium. The lymphatic grows by 

 the sending out of fine protoplasmic processes which become 

 definite lumen-containing sprouts. Nuclear areas in the sprout 

 are provided by the in-wandering of nuclear areas from the main 

 stem, and by mitotic division. New lymphatic protoplasm and 

 nuclei, therefore, are formed by the extension of preexisting lym- 

 phatic endothelium . In its peripheral growth the lymphatic 

 endothelium is not formed by the transformation of mesenchyme 

 cells or blood vessels, nor does it give rise to mesenchyme cells or 

 blood vessels, it is a specific independent tissue. Throughout 

 its growth, the endothelial wall of the lymphatic capillary is 

 closed, there are no open communications between the lumen of 

 the lymphatic capillary and mesenchyme spaces. 



