LYMPHATICS IN FROG LARVAE 11 



The early lymphatics follow so closely along the underlying 

 veins and in the adjacent areas that their distribution and 

 relations are readily made out against the deeper pattern of 

 the veins in these simple forms. 



The presence of the lymphatic system supplemental or 

 accessory to the veins is not yet fully explained, but the rela- 

 tions of the lymphatics to the peripheral connective tissues 

 and to the pronephros, as exposed by injection in the larvae 

 of Amphibia, is so suggestive of a causal connection that 

 special attention to these relations and experiments on the 

 early stages of these forms promise interesting data toward 

 the solution of the problem of the fundamental function or 

 meaning of the lymphatic system in vertebrates. 



In the higher vertebrates, as mammals in which the lym- 

 phatics have received so much attention, the tissue cells adja- 

 cent to the lymph vessels increase rapidly, and overshadow 

 the primary association between lymphatics and veins, with 

 consequent changes in both anatomical and physiological con- 

 ditions. Hence it seems important to examine with particular 

 thoroughness the vessels of the lower vertebrates in initial 

 stages when unobscured by tissue cells. 



In addition to data obtained from the study of vessels, such 

 a review of other structural characters will help to establish 

 a much needed standard of reference in these early growth 

 stages, other than that of length, which proves to be quite 

 unreliable, both between specimens from different species and 

 those of the same species. Thus, for instance, the 6-mm. 

 embryo of palustris is no further advanced anatomically than 

 a 4- to 4.5-mm. toad. Hence this summary, and the need of 

 a similar check on later stages. 



These prelymphatic, or primary lymphatic, larvae are short 

 and straight, with total length of only about 6 mm. in R. 

 palustris, 2 mm. of which is tail (Pollister's early 21, '37). 

 In side view, figure 1 (figures are to be found on plates at 

 end) the organs are seen to be arranged in three zones; 

 the neural tube dorsally, the gut tube ventrally, and between 

 these, in the middle zone along the axis, the notochord, with 

 the lateral muscle segments. The body is compressed from 



