LYMPHATICS IN FROG LARVAE 25 



So, attempting to overcome the great technical difficulties 

 of these stages, prolonged effort was devoted to obtaining the 

 necessary special injections for definite pictures of the details 

 of structure of the primary lymph plexus itself, and at the 

 same time to make clear its relations to the veins of the seg- 

 ments in which it is first established. 



From this collection the injections illustrated in figures 3 

 and 4 are presented first, 2 since in both specimens the lym- 

 phatics are filled in contrast to the blood vessels, and thus 

 make a special demonstration of the primary lymph plexus. 

 These larvae are of the same stage as that of figure 2. 



Exposed in this way the early lymphatics are seen in figure 

 3 as a well-defined small-meshed plexus of irregular outline, 

 readily distinguished from the segmental veins and super- 

 ficial to these in the region dorsal to the pronephros. The 

 three anterior intersegmental veins are shown emerging from 

 beneath the ventral border of the plexus, adjacent to it. Still 

 deeper, dorsal arteries (Dsl.A.) and blood vessels on the cord 

 (N.Plx.) are faintly visible. Several delicate lymphatics run 

 from the dorsal edge of the plexus over the cord. The poste- 

 rior margin of the plexus overlaps the fourth segment. 



Though connections undoubtedly persist (text fig. A) be- 

 tween the lymphatics and the veins deep to them, the main 

 pathway from the lymphatic plexus is into the anterior lymph 

 heart and through it to the third intersegmental veins, as in 

 the first stage (fig. 1). 



In figure 3, the vessels of the plexus converge to ducts con- 

 necting with the lymph heart. From this parent group the 

 plexus has spread out beyond its primary limits, an anterior 

 section extending forward in a double line; the dorsal vessels 

 lateral to the hind-brain ending blindly back of the ear; while 

 the ventral line forms a plexus over the dorsal surface of 

 the pronephros, seen in the picture over the first interseg- 

 mental vein. 



1 Successful injections in this period are rare. The tendency of the injection 

 to avoid the delicate vessels of the lymph plexus and run instead into larger 

 vessels which may be near was overcome in these cases by introducing the needle 

 close to the heart. 



