LYMPHATICS IN FROG LARVAE 47 



Accelerated growth in structures of the tail connected with 

 initiation in this period of greater activity in swimming, now 

 becoming habitual, leads to increase in the musculature and 

 broadening of the fin folds, creating new conditions in the 

 region, which add stimulus to the development of both blood 

 vessels and lymphatics. 



Differentiations in lymphatic system in the third period 



Injections of the lymphatics in this period exhibit altera- 

 tions which make a distinct difference in the appearance of 

 the system in both the central plexuses and their peripheral 

 extensions. Centrally, dorsal to the lymph heart, definite 

 longitudinal ducts (Dsl.R.D.) have taken the place of the 

 irregular plexus of the previous stage. These long ducts are 

 not new formations, but arise by transformation of functional 

 paths established through the plexus of the early period, as 

 is indicated by comparison with injections like that shown in 

 figure 5. In this specimen, as pointed out in connection with 

 the description of the figure, while the plexus is still an intact 

 network, two paths are demonstrated through it by the in- 

 jection fluid: one in the dorsal margin of the net, the other 

 ventral and parallel with this; the two paths anticipating, in 

 their relations and connections, the ducts found over the heart 

 in the older larva now described (figs. 13, 14, 15, etc.). 



Hence, when the channel along the dorsal margin of the 

 plexus in the earlier stage becomes fixed as the dorsal duct 

 of the older larva, it may be identified as the main 'dorsal 

 reception duct' for the heart (Dsl.R.D., figs. 13, 14, 15). It 

 may be so-called since it not only retains connections with the 

 peripheral lymphatics from dorsal structures, but is joined 

 by the collecting duct from the lateral lymphatics with lymph 

 from the posterior systems ; and, besides, receives lymph from 

 in front, through distal connections with the ventral second- 

 ary duct from the anterior body wall and head. 



In the early stage for the period (frog, fig. 13) a relatively 

 large amount of the lymph from the front reaches the lymph 

 heart, and there are, temporarily, separate openings in the 



