378 ELIOT R. CLARK 



they gradually resume the character of the non-dividing nuclear 

 areas. During the division the wall of the lymphatic remains 

 continuous. 



Mitotic divisions have been observed in the main caudal lymph 

 trunk as well as in the branches. There seems to be no definite 

 proportion between the number of nuclear areas a branch may 

 receive by in-wandering as compared with the increase by divi- 

 sion. I have seen the first nuclear area in a sprout divide, one 

 of the daughter nuclear areas remaining, and the other passing 

 back to the main stem. On the other hand, I have seen as many 

 as five nuclear areas wander into a branch. 



Thus it is found to be possible to keep an accurate account of 

 all the nuclear areas in a growing lymphatic sprout, from its very 

 beginning. When this is done it is found that the sprout receives 

 its nuclear areas in two ways — by the in-wandering along the wall 

 of nuclear areas, which were present in the parent stem, and by 

 mitotic division of those which have wandered in. Moreover all 

 the nuclear areas which have been watched remain as nuclear 

 areas of the lymphatic. I have not seen them form leucocytes, 

 red blood cells, or mesenchyme cells. The sprout, then, receives 

 its nuclear areas from the preexisting endothelium. That the 

 protoplasm of the sprout is also derived from the preexisting 

 endothelium was shown in an earlier paper. 7 



What, now, are the relations between the growing lymphatic 

 and mesenchyme cells, which abound in the region which the 

 sprout is invading? In all my studies it has been observed that 

 the lymph sprout is so far from adding to itself these cells that it 

 actually remains at as great a distance as possible from the main 

 bodies and larger processes of the mesenchyme cells. At the tip 

 of the sprout, long fine processes are sent out by the lymphatic 

 in various directions. Occasionally such a thread pushes toward 

 or even to the body of the mesenchyme cell. But in all instances, 

 according to my observations, this process is eventually with- 

 drawn and the path selected is, as said, midway between these 

 cells (cf. the relations between lymph sprout and cell J, in figs. 



7 E. R. Clark, 1. c. 1909. 



