74 HENRY MCELDERRY KNOWER 



ever, many injections made in larvae of R. palustris to deter- 

 mine the time and mode of development of the posterior lymph 

 hearts do not support Hoyer's insistence that these hearts 

 arise as evaginations of the wall of the posterior vertebral 

 vein, and connect with the already established primary lymph 

 plexus by outgrowths from the heart walls. 



On the contrary, the present injections agree rather with 

 the reconstructions of Kampmeier of the developmental stages 

 of the posterior lymph hearts in the toad, insofar as the 

 hearts are certainly proved to arise from lymph vessels of 

 the lateral caudal plexus on the base of the tail. However, 

 Kampmeier 's account for the toad of a further series of ad- 

 justments of the hearts in the plexus has not been confirmed 

 in the injections of frog larvae. 



Special injections of the region for both lymphatics and 

 blood vessels discover no lymph hearts before the 12-mm. 

 stage of R. palustris; but then, unexpectedly, and contrary 

 to the generally accepted opinion that these hearts are not 

 found until after the appearance of posterior limb buds, small 

 lymph hearts may be demonstrated in this much earlier stage, 

 in the close-meshed net of the lateral caudal plexus. 



One of these injections of the first stage of the posterior 

 lymph hearts (R. palustris, 12 mm.) is illustrated in figure 33; 

 the two little hearts evidently merely dilated vesicles at the 

 junction of lymph capillaries of the finer plexus along the 

 lateral line, just ventral to the lateral caudal collecting duct. 

 The hearts in these injections are thus units from the first, 

 and not the result of a coalescence of several vessels as in the 

 toad; though their character, as derivatives of areas of the 

 lymph plexus, seems definitely established, both by the re- 

 constructions of the toad and in these injections. 



When, however, the problem of the exact relationships of 

 the posterior lymph hearts in the lymph plexus and to the 

 veins is approached, disagreement among authors is extreme 

 and most difficult to resolve. On the one hand, Hoyer ('34) 

 adheres to his original view based on the work of Baranski 

 (11) and Fedorowiczs ('13) that these hearts arise as out- 

 growths from the wall of the posterior vertebral vein, in 



