GROWING LYMPHATICS AND THE MESENCHYME 355 



weeks without seriously interfering with heart action or the 

 growth of the tissues. The anesthetization of the other species 

 offers many difficulties. Often from a half-hour to an hour or 

 even more may be taken up in the attempt to bring the animal 

 to the proper point of anesthesia, where muscular movements 

 are lost and the heart beat is vigorous. Occasionally the heart 

 beat stops entirely. In such a case it will usually be resumed as 

 the result of a delicate massage of the heart produced by rhyth- 

 mically forcing a stream of water against the heart region with a 

 medicine dropper. The difficulties of anesthesia increase as the 

 larva grows older. It was thought that this might be due in 

 part, at least, to the greater need for oxygen. Whether this 

 explanation is true or not, certain it is that frequent changing 

 of the chloretone solution greatly assisted the anesthesia. This 

 is most conveniently done by employing the modified apparatus 

 described, with which a continous circulation of the chloretone 

 solution may be kept up. During the intervals between obser- 

 vations the larva is returned to fresh water. 



A short review of the results of the observations previously 

 reported will now be given. It was possible to see clearly the indi- 

 vidual structures present in the fin expansion of the tail. Lym- 

 phatics, blood vessels, nerves, mesenchyme cells with their 

 branched processes, red and white blood cells, stand out with 

 remarkable clearness. The immobility of the object, which may 

 be maintained for hours, makes possible both prolonged obser- 

 vations of a small selected area, as well as careful records of small 

 or large areas. It is possible to watch, from minute to minute, 

 the changes which go on in a selected cell or process, and it is 

 also possible to make accurate drawings of the entire blood- 

 vascular or lymphatic plexus of both fin expansions. 



Among the results of this first series of observations, in which 

 the structures were studied both extensively and intensively, 

 it was found that lymphatics grow by a process of sprouting, in 

 which, so far as concerns the material of which their endothelium 

 is formed, they maintain a complete independence of blood ves- 

 sels, mesenchyme cells, and wandering cells. In the species 

 studied the blood vessels precede the lymphatics, in their invasion 



THE AMERICAN JOURNAL, OP ANATOMY, VOL. 13. NO. 3 



