LYMPHATICS IN FROG LARVAE 19 



The dorsolateral plexus connected with the anterior lymph 

 heart may vary considerably in extent in different specimens 

 on account of the difficulties, already explained, of injecting 

 the first segments of this stage ; but comparison of numerous 

 injections of the period reveals that when the lymph heart 

 first appears this special group of vessels develops at the same 

 time, associated with it as local out-growths of the general 

 superficial blood vessels of these segments with which they 

 remain connected through early stages. As figure 1 shows, 

 then, the anterior lymph heart is actually the first lymphatic 

 structure to be identified in the plexus. It empties into the 

 third intersegmental vein near its ventral end. 



While the special features of the first four segments related 

 to the lymph heart concern us most, the general vascular 

 arrangements of these segments are also well shown in the 

 drawing (fig. 1), and agree with the pattern found throughout 

 the rest of the dorsolateral systems, though accessory periph- 

 eral loops are lacking in the first segments. The three first 

 veins are well established, their rootlets beginning from the 

 lateral aspect of the neural plexus, and, like the rest of the 

 series of intersegmental veins, run immediately outward over 

 the dorsal edges of the lateral muscle sheets. Prom this 

 dorsolateral line the collecting veins run down on the outer 

 surface of the muscles receiving deep branches from the 

 muscles in their course to the pronephric sinus which lies 

 along the ventral borders of the muscles. 



In passing vent rally from the dorsal plexuses, the inter- 

 segmental veins not only receive veins from the deep muscles, 

 but are connected as well by small well-defined vessels run- 

 ning between them lengthwise in an irregular plexus over the 

 outer surface of the muscles. This is the general superficial 

 plexus mentioned above, and is best illustrated in the picture 

 in the two anterior segments, the injection behind these having 

 run almost exclusively into the smaller lymph heart plexus 

 around the third vein. 



It should be clear, from the above description, that no lym- 

 phatics are to be found outside of the first four segments in 



