48 HENRY MCELDERRY KNOWER 



dorsal wall of the heart for the two (dorsal and ventral) 

 ducts. Later a larger proportion of lymph from the anterior 

 system is diverted into the main dorsal duct (fig. 14) until 

 soon the whole supply passes through this vessel; as on the 

 left side of the same specimen (fig. 15) and in older larvae. 



LATE LARVAE (STAGE 4) OF THIRD LYMPHATIC PERIOD 



It is important to note that, in the process of transforma- 

 tion from the plexus to the duct which substitutes for it, the 

 afferent openings (portals) into the dorsal wall of the heart 

 are maintained without break in continuity, and free com- 

 munication between the heart and the permanent larval col- 

 lecting duct arching over it is demonstrated also in later 

 specimens (figs. 13, 14, 15, etc.). 



The posterior lymphatics of the dorsal and lateral systems 

 exhibit much activity in this period. A change from the 

 former segmental pattern in the posterior lymphatics to a 

 more strictly longitudinal arrangement in both dorsal and 

 lateral plexuses, begun in this period, is an adjustment to an 

 increasing flow of lymph from accelerated growth of struc- 

 tures in the tail. As a result of this new stimulus, the main 

 collecting ducts in both dorsal and lateral (and ventral) plex- 

 uses, which from the first provided the most direct path to 

 the lymph heart and pronephros, become gradually enlarged 

 and more prominent, at the expense of segmental connections. 

 This is evident in all injections of this stage, as illustrated in 

 figures 13, 14, 15 and 16. 



The long dorsal and ventral caudal vessels (the ventral 

 vessels are not completely injected in these specimens) quickly 

 reach the tip of the tail, and produce processes along their 

 course which grow out to invade the adjacent lateral tissues 

 and the fin folds. Figure 13 (frog, stage 3) illustrates the 

 early stage of this growth, and shows the long terminal fila- 

 ments by the growth of which the dorsal lymphatic has been 

 extended to the end of the tail. Lymphatic fin processes and 

 lateral outgrowths are quite short in this stage, and only a 

 few are represented in the drawing. The primary lateral 



