LYMPHATICS IN FROG LARVAE 71 



body wall and their relations to the lymph heart in the later 

 stage, illustrated in figure 28, are largely confined to increase 

 in size with better differentiation of the proximal ducts. Thus 

 the anterior plexus is extended further into the body wall, 

 beyond the pronephric area in figures 23, 24, 25, 26, 27, 28, 

 while the afferent ducts are stronger and more definitely 

 organized. 



The details of the developmental changes in the lymphatic 

 system, described above for the late 18-mm. larva of R. palus- 

 tris, and illustrated in figures 30 and 31, will be useful in 

 obtaining an understanding of a number of modifications in 

 the system found in Hover's well-known pictures of the older 

 26-mm. larva of R. temporaria. It would be impossible to 

 trace, throughout the series, continuity with the relations of 

 the early stages without the intermediate conditions here dem- 

 onstrated in the 18-mm. R. palustris larva. 



Comparison of the data presented in figures 30 and 31 of 

 the present paper with Hoyer's figures of the older larva 

 (26-mm. R. temporaria, '05), which are hero reproduced as 

 text figures D and E for that purpose, will explain how tin- 

 later conditions arose. 



SUMMARY OF ADJUSTMENTS OP LYMPHATICS WHICH 



PRODUCE THE DEFINITIVE ARRANGEMENT 



DESCRIBED FOR HOYER'S LATE 



FROG LARVA 



A list of the main points will probably suffice here. 



1. More exact and detailed relations of the lymph heart to 

 the afferent lymphatics immediately surrounding it than are 

 disclosed in the later stage are furnished in the injections of 

 the 18-mm. R. palustris larva and preceding stages. These 

 prove that the over-arching dorsal duct of Hoyer's later speci- 

 men arises secondarily by differentiation of a tubular vessel 

 in the primary lymph heart plexus. 



As the result of alterations described in connection with 

 figures 5, 13, 14, 23, 27, etc., for the differentiation of the heart 

 plexus, anterior and posterior collecting ducts are formed, 



