GROWING LYMPHATICS AND THE MESENCHYME 359 



since areas may be selected in which no lymphatic or blood vessel 

 has appeared. 



The exact method chosen for making the desired studies was 

 as follows. In the tail of the tadpole the lymphatics first appear 

 as two longitudinal vessels, which have been many times described, 

 the dorsal and ventral caudal lymph trunks. In hyla pickeringii 

 the dorsal trunk, save for a short distance at the tip of the tail, 

 is concealed between the muscle layers. The ventral caudal 

 lymph trunk, on the other hand ; may course for considerable 

 stretches, even for its entire extent out from under the muscle, 

 so that it may be clearly seen. The caudal vein is situated just 

 dorsal to the lumph trunk, while, further dorsally, ventral to the 

 notochord, runs the caudal artery. The vein may be seen quite 

 readily, especially toward the tip of the tail, where the muscle 

 layer is thinnest. A stage may be selected in which neither 

 blood nor lymph vessel has grown from the main trunks into 

 the ventral fin. 



It was planned to select for study a portion of the ventral caudal 

 lymph trunk, at this early stage, before any branches had been 

 sent out, to make a careful record of each individual mesenchyme 

 cell around it, as well as of each mesenchyme cell in the entire 

 region in which a sprout of the lymphatic would be expected to 

 grow, and, in case a sprout were sent out in the region hoped, 

 to make numerous successive studies and records of each indi- 

 vidual mesenchyme cell, as well as of the lymphatic, as they 

 developed side by side. 



My hopes were amply fulfilled. In the selection of a portion 

 of the lymph trunk, a place was chosen where a slight ventral 

 bend occurred, as shown in fig. 2. The larva, at this time, was 

 about 5.5 mm. long. At this spot the lymphatic was well away 

 from the muscle edge, so that both walls of the lymphatic were 

 clearly visible. It was also well separated from the caudal vein. 

 The vein was easily recognizable through the very thin muscle 

 layer. A drawing was made of the lymphatic, vein, muscle edge, 

 and of the neighboring mesenchyme cells, with their main proc- 

 esses. The tadpole was then placed in fresh water over night. 

 The following morning a lymphatic sprout had started to grow 



