LYMPHATICS IN FROG LARVAE 45 



A more detailed comparison of the results of the two 

 methods discussed above is not possible; but the major fea- 

 tures here compared indicate that a choice of two alternatives 

 is presented: either important differences of the reconstruc- 

 tions from injections aue the result of technical difficulties in 

 applying the reconstruction method in toad larvae (partly by 

 missing connections between units and partly by mistaking 

 tissue spaces for veins or lymphatics in sections) ; or, appar- 

 ently, it must be concluded that the origin and establishment 

 of the lymphatic system in the toad is exceptional, and follows 

 a retarded secondary method not typical for Anura. This 

 conclusion would be based on the relatively late origin of the 

 lymphatics in the toad in separate centers by different 

 methods ; and by their delay in coming together to unite in a 

 single system. An added atypical feature is the breaking of 

 connections between the lymph heart and its surrounding 

 afferent vessels in late larvae, with further delay in the final 

 establishment of the system. 



THIRD LYMPHATIC PERIOD (STAGES 3 AND 4) 



LARVAL CHARACTERS AND PERIPHERAL DIFFERENTIATION OF 



LYMPHATICS 



Larval structures. The establishment of the primary lym- 

 phatic system discussed in connection with figures 2 to 12 for 

 the larva of the second lymphatic period is followed by rapid 

 growth and differentiation, resulting in distinctive characters 

 in both the larva and the lymphatic system which mark a new 

 period of development. Among these larval changes certain 

 new features which appear in the digestive tract connected 

 with the formation of an S-shaped coil may be especially re- 

 lied upon as guide-marks to the progress of development. 

 Thus, early in the second day of the new period, a decided 

 kink or bend from right to left appears in the hind-gut, mark- 

 ing the beginning of the intestinal coil and the third stage of 

 the series examined in the present paper, illustrated in figure 

 13 (Liu-Li's stage 2, '30; Pollister's early 22, '37). 



Some 12 hours later, the entire yolk-filled digestive tube 

 has become involved in the S-shaped coil thus started, and 



