LYMPHATICS IN FROG LARVAE 57 



lymph hearts, there is a growing tendency in the vessels 

 around them to distend as if from too slow evacuation. This 

 dilatation is evidently beginning in stage 3 (fig. 13) where the 

 anterior lymphatics are all somewhat enlarged. In the follow- 

 ing stages much more fluid is collected by the lymphatics of 

 the tail, greatly increasing the flow forward into the vessels 

 around the heart, where it meets the relatively smaller and 

 weaker contributions from the head. 



It appears probable, then, that the great distention of the 

 lymphatics to form sinuses in the head, first exhibited in later 

 specimens of this period in figures 14, 18, 19, 21, etc., is due 

 to a mounting congestion in the lymphatics around the ante- 

 rior lymph heart and immediately in front, produced by a 

 greater flow of lymph from behind than the system can elimi- 

 nate or adjust to. Escape of lymph into these vessels from 

 the narrow jugular duct in front is apparently checked by the 

 resistance of a rising pressure opposed to the weak contribu- 

 tion from the temporal vessel (sinus). The result is marked 

 and continued distention of the temporal sinus and its distal 

 enlargements in the maxillary region. 



Since these conditions persist and are even exaggerated in 

 later stages, the sinuses of the head become steadily larger 

 and more conspicuous. (See figs. 19, 24, 30, etc., for the tem- 

 poral sinus in later stages.) 



The same influences which affect the lymphatics of the head 

 so extremely may also be assumed to bring about important 

 adjustments in the vessels back of the anterior lymph hearts. 

 But here, with the vessels spread out on the broad surface of 

 the lateral muscles, the results appear much more gradually. 

 The tendency of the middle group of the lateral lymphatics 

 to gather lymph along the lateral line, already marked in 

 stage 3, becomes constantly more pronounced until finally the 

 increased flow here establishes a large distended trunk along 

 the dorsal wall of the body cavity, to be identified as the 

 'lateral body lymphatic' (figs. 26, 27, 30, 31, etc.). This trunk 

 is evidently formed by the gradual enlargement and fixation 

 of the earlier pathway through the lateral plexus, the steady 



