LYMPHATICS IN FROG LARVAE 27 



of later stages. Thus two divergent series of vessels are 

 formed ; one along the hind-brain to end above the ear vesicle ; 

 the other a similar irregular outgrowth (jugular lymphatic) 

 ventral to the ear in the dorsal wall of the branchial cavity. 

 The magnified picture in figure 4 demonstrates the character- 

 istic irregular branches and fine terminal tips typical of these 

 peripheral lymphatics. 



The distal plexus in the body wall over the pronephros, 

 usually conspicuous, does not appear in this injection. The 

 jugular duct, however, is seen as an irregular plexiform chan- 

 nel, running a short course forward in the dorsal wall of the 

 branchial cavity, readily identified in early injections since 

 no other vessels are found in the same location. Distally it 

 unites with the collecting duct from the anterior vessels of 

 the body wall to form a combined duct which can be followed 

 in dorsally to junction with other afferent lymph ducts in the 

 'anterior confluence' (Ant.Confl.) and to final connection with 

 the lymph heart plexus (fig. 4). It should then be clear, 

 from study of this injection, that the main path for lymph 

 from the periphery to the lymph heart is indirect through 

 dorsal connections with the lymph heart plexus, and that a 

 diversion through ventral ducts to the heart is a transient 

 feature, not affecting the establishment of the definitive larval 

 system. 



While the primary lymphatic plexus is thus differentiating, 

 the veins of the same segments develop additional features, 

 increasing their areas of drainage, yet maintaining their 

 original relations. 



The anterior segmented reins in the second period 

 From their first appearance in injections, the three anterior 

 intersegmental veins are important features in the drainage 

 of the hind-brain and cervical cord, and of the lateral muscles 

 of these segments. They form a well-established venous 

 plexus at this time in figure 1. 



In the second period (fig. 2) the same relations persist, and 

 these veins become more permanent as the flow of blood 



