LYMPHATICS IN FROG LARVAE 77 



Injections of early posterior lymph hearts reveal them on 

 the surface of the muscles, ventral to the main lateral caudal 

 vein. The hearts are in association with the deeper capillary 

 branches of the vein, well outward from its wall. Several 

 small lymphatic processes can generally be traced from neigh- 

 boring lymph trunks to the heart through the superficial 

 mesenchyme; but it has not been possible to demonstrate, 

 crucially, an efferent connection of a lymph heart with a 

 venous capillary, though the evidence of many injections 

 points strongly to the presence of such an early connection, 

 rather than a direct entrance from the heart into the large 

 vein. Since most injections avoid the capillary connections, 

 and run around, entering either only hearts and lymph vessels 

 or, instead, veins alone, the connections between small vessels 

 of the two systems are extremely difficult to demonstrate. 

 The most satisfactory specimens are those with the veins in- 

 jected in one color (either black or blue), and the lymphatics 

 filled with another color, on which the hearts appear as en- 

 largements. 



In later stages, when the lymph hearts are larger, they 

 have drawn closer to the larger vein, and become directly 

 connected with its wall, a secondary relation. 



More definite specimens of this process are needed, but from 

 the injections in hand, I believe that the lateral lymph vessels 

 of the superficial caudal plexus which is demonstrated before 

 the appearance of posterior hearts, as in figure 32, are already 

 connected deeply with the underlying small veins over the 

 caudal muscles. Undoubtedly the two superficial plexuses of 

 veins and lymphatics spread and develop independently, but 

 their early connections have been demonstrated, and I have 

 seen them again in certain areas of later stages. The poste- 

 rior lymph hearts are then later local pulsatile enlargements 

 of small lymph vessels (fig. 33) in such areas of connection 

 with small veins beneath. Like the anterior lymph hearts, 

 the posterior series are to be regarded as local reactions to 

 influences between the two plexuses, focussed at the critical 

 points where they arise. These conclusions follow closely 



