LYMPHATICS IN FROG LARVAE 35 



The example of uniform distribution of the injection in the 

 plexus illustrated in figure 9 shows the vessels arranged bi- 

 laterally in two well-defined lines, running forward parallel 

 to the mid-dorsal line, to end irregularly in front. This in- 

 jection is similar to that shown in figure 5, and, as in that 

 specimen the fluid, coming forward, is diverted on each side 

 to the lymphatics back of the lymph heart through posterior 

 connecting vessels, the anterior vessels of the plexus and their 

 connections being left incomplete. 



In contrast to the type just described, figure 10 represents 

 a common variation (perhaps the most usual) brought out by 

 the establishment of a path of low resistance along the mid- 

 line of the plexus, the vessels here filling excessively to crowd 

 together from the two sides as if to fuse. But fusion does 

 not occur, the main features of the bilateral arrangement per- 

 sisting essentially the same as in figure 9. 4 



The dorsal lymphatics in this injection, like those in figure 

 6, retain the primary arrangement, extending far forward 

 over the anterior lymph hearts and connecting through their 

 lateral vessels in a series of branches beginning over the 

 anterior segments. 



A unilateral type is illustrated in figure 11, in which a long 

 trunk is conspicuous along the right side of the cord with 

 strong connections injected through the plexus across the mid- 

 dorsal line to join the left heart plexus. 



Figure 12 represents another injection of this period, illus- 

 trating the early tendency to the formation in the primary 

 plexus of paths w r hich resemble or anticipate definitive pat- 

 terns, though these patterns are not established until a de- 

 cidedly later stage (as in figs. 23, 26). Tn this case (fig. 12) 

 the lymph from the tail runs forward in a conspicuous mid- 

 dorsal track which passes through the anterior plexus to 

 connect laterally with the lymph heart vessels on each side, 



4 When these two injections are compared in each segment the S-shaped vessel 

 in front followed by a loop in figure 9 is duplicated, though obscurely, in figure 10 ; 

 while in the next two segments, posteriorly, the parallel vessels seen in figure 9 

 are brought close together in the mid-line of figure 10, but remain separate. 



