76 HENRY MCELDERRY KNOWER 



hearts grow steadily larger, with stronger interconnections, 

 and their increasing activity accelerates the flow of lymph 

 through those lymph ducts of the plexus which converge 

 toward the hearts along the lateral line where they connect 

 with intersegmental veins. 



Figure 34 illustrates an early phase of the enlargement of 

 the posterior hearts and of their vessels in an 18-mm. larva 

 of E. palustris with first hind limb buds, like the less magni- 

 fied figure 30. The influences just described, exerted by the 

 hearts on the lymph plexus, are evident. It will also be noted 

 that the lymph vessels of the plexus are beginning active pro- 

 duction of small processes from their walls at this stage, 

 especially around the hearts. 



Further progress in the growth and adjustment of the 

 caudal lymphatics and posterior hearts to late conditions is 

 shown in figure 35, for a larva with legs in mid-development, 

 not yet extended. The larger lymph vessels have here ac- 

 quired a segmental arrangement, and are strongly marked 

 along the veins. The lateral vein is lightly drawn, deep to 

 the hearts, which connect with its segmental branches. 



Many processes now extend out from the walls of the lym- 

 phatics everywhere, forming especially intimate association 

 with the walls of the hearts. The enlargement of the small 

 fourth heart on its first appearance, shown in the inset to the 

 right in figure 35, illustrates the complex association of proc- 

 esses from the surrounding lymph vessels with the heart. 



The photograph of the caudal lymph plexus of a later larva, 

 with hind limbs fully extended, in figure 36, is introduced to 

 show the character of the extensive network formed by the 

 anastomosing processes, in the latest larval stages. 



It should be noted that, in these late stages (figs. 34, 35, 36), 

 the processes which multiply around the hearts grow toward 

 them, rather than in the reverse direction, outward from the 

 hearts, to spread over the muscles as Hoyer suggests. These 

 centrifugal processes from the adjacent lymphatics which 

 penetrate the walls of the posterior lymph hearts in the late 

 period are probably the 'detached cell strands' of Fedorowicz 

 and the rudimentary lymph vesicles of Huntington. 



