LYMPHATICS IN FROG LARVAE 63 



Formation of definitive lateral lymph trunks 



In the earlier larvae, the ventral and lateral trunks (figs. 

 14, 15, 23, 26) are well-defined vessels in the lateral plexus, 

 through which they run in converging paths to their junction 

 anteriorly. Views of two sides of specimens (fig. 24, the left 

 side of 23; and fig. 27, dorsal and left lateral view of 26) 

 illustrate the contrasts acquired with the meshes of the plexus 

 by the larger vessels as they run forward to the anterior 

 lymph heart. 



In the further differentiation (during Pollister's stage 23) 

 the lateral and ventral ducts gradually draw together in the 

 lateral plexus to fuse anteriorly in a single large vessel, the 

 'lateral body lymphatic' of late stages, while the connecting 

 vessels of the plexus are atrophied or incorporated in the 

 large trunk. Accordingly, the series of segmental connections 

 from the dorsal lymphatics run laterally to join this large 

 body lymphatic in dorsal view of late stages instead of follow- 

 ing their original dorsoventral course. 



During the period of fusion, the lateral and ventral lymph 

 trunks lie together through much of their course in a section 

 of the plexus still left within the posterior wall of the abdomen 

 (figs. 29, 30), and at this stage the ventral lymphatic can be 

 traced up from the base of the tail along the curved abdominal 

 wall. As it approaches the abdominal wall the ventral caudal 

 lymphatic, like the dorsal duct, is seen in figures 29 and 30 

 to receive contributions of lymphatics from deep and super- 

 ficial neighboring structures of the tail. Entering the poste- 

 rior body wall as the 'ventral body lymphatic,' it receives 

 afferent ducts from around the rectum and the ventral body 

 wall, and then passes upward through the tissues of the poste- 

 rior body wall in the course described. 



After the hind limb buds appear, additional ducts grow out 

 to drain them. 



If some of these relations are hidden, they may be demon- 

 strated by removing the overhanging body wall (fig. 29). 



As in earlier stages (figs. 13, 15, 24, 26), the 'lateral body 

 lymphatic' is continued forward from a plexus (the 'lateral 



