LYMPHATICS IN FROG LARVAE 49 



connections with the reception ducts which replace the lymph 

 heart plexus persist in front. 



Following figure 13 an excellent picture of progress in the 

 lateral lymphatics is given in figures 14 and 15 for the later 

 stage (4) of the period. The plexus is here seen to extend 

 further posteriorly over the veins, which are represented in 

 straight smooth outline in the drawings, while the lymphatics 

 are irregular with delicate processes on their walls. The 

 vessels of the anterior or older section of the plexus are 

 already larger and coarser than the rest, and the strongest 

 connectives from the dorsal lymphatics join this group in 

 front. The ventral lymphatic formed in younger larvae as 

 an outgrowth from the lower vessels of the plexus is well 

 shown in the pictures, its junction with the lateral vessels just 

 back of the heart especially well defined in figure 15. 



The stimulating effect of active development in the tail in 

 this period, on both blood vessels and lymphatics, is illus- 

 trated in figures 14, 15, 16 and 17, where the plexuses of the 

 blood vessels in the fins are shown extended well beyond the 

 limits of the inner zone of the last stage associated with 

 lengthened peripheral processes of the caudal lymphatic. 



In figure 14 the main dorsal lymphatic is seen to have grown 

 out to the end of the tip of the tail and its processes in the 

 fins, with those in the lateral tissues, have elongated consider- 

 ably. (The injection has not completely filled the ventral 

 trunk in this case, but its development is like that of the 

 dorsal vessel.) The details of structure of the peripheral 

 lymphatics, similar to the earlier peripheral processes shown 

 in figures 5 and 13, are drawn in figure 17, which furnishes a 

 magnified view of the end-piece of the specimen shown in 

 figure 14 under low power. The characteristic irregular rami- 

 fications, delicate terminal points, and fine lateral projections 

 from the walls of such vessels are brought out in the enlarge- 

 ment. Many small beginning sprouts with fine points are 

 observed, and a tendency toward plexus formation is evident 

 in the longer branches (T.Fl.Dsl.L. and T.L.Dsl.L.). 



