GROWING LYMPHATICS AND THE MESENCHYME 375 



base of the branch for a short distance and then retreat, to con- 

 tinue their movement along the main stem. (fig. 16, nuclei 

 2* and 2 h in stages F to J). Moreover, often in the developing 

 lymph system, a branch which is quite active at one time, is 

 apparently found to be superfluous later. It then becomes 

 narrower and shorter and the nuclear areas, which may be present, 

 retreat to the parent stem. 



These nuclear movements furnish strong evidence for the view 

 that the endothelium of the new lymph sprouts is a syncytium. 

 That one nucleus could move past another and that such changes 

 in relative position as are shown in fig. 16 could take place, if 

 there were definite cell boundaries seems hardly conceivable. 

 Many observers have tried in vain to find, by the use of silver 

 nitrate, outlines of endothelial cells in the lymphatic sprouts in 

 the tail of the frog larva. I have injected them directly with 

 silver nitrate, but have failed to find endothelial markings. More- 

 over, the study of the wall of these capillaries in toto, in alcohol, 6 

 as well as in sections stained with intense protoplasmic stains 

 such as acid fuchsin, reveals a network of fibrillae which connect 

 neighboring nuclear areas, thus forming a syncytium. 



The other striking property possessed by the nuclear areas, 

 which has been observed in the living larva, is that of mitotic 

 division. Here, as in the case of the mesenchyme cells described 

 above, the picture was unmistakable (fig. 17). The granules 

 disappear, as the central portion becomes clear and spindle- 

 shaped. As in the dividing mesenchyme cells spireme formation, 

 arrangement of chrosomes, formation of daughter nuclei, and the 

 cutting apart of the remainder of the cell are to be seen with sur- 

 prising clearness. The chromosomes separate so rapidly that 

 the actual movement may be seen. The cutting-in of the cell 

 also takes place rapidly — from one to two minutes only being 

 consumed. After division the two daughter nuclear areas move 

 apart and maintain for a time a characteristic shape. Later 



6 E. R. Clark, An examination of methods used in the study of the development 

 of the lymphatic system, Anatomical Record, vol. 5, no. S, 1911, p. 403, fig. 1, 

 and p. 406. 



