LYMPHATICS IN FROG LARVAE 81 



though faintly drawn cord plexus, with the contrasting tribu- 

 taries of the anterior vertebral vein, form an excellent picture 

 of this system in which the main vein is now prominently 

 differentiated as it joins the third intervertebral vein. 



The anterior plexus of the body wall, over the pronephros, 

 is becoming segregated around the future cutaneous vein, 

 which is not yet determined in this stage, but will run forward 

 medial to the great cutaneous artery. The facial connecting 

 veins are strongly developed as they pass over the cutaneous 

 artery to the first intersegmental vein. The beginnings of 

 the great abdominal vein extend forward from the ventro- 

 lateral margin of the cutaneous plexus. 



The lymphatics of this period, illustrated in figures 23 to 27, 

 have the same relations to the blood vessels as in previous 

 stages in which they are exposed to view. Their location is 

 indicated by the position of the anterior lymph heart on the 

 third vein. The proximal ducts of the central lymph system 

 are also found here, but the lymphatics are now so overgrown 

 by the blood vessels as to require separate study of the two 

 systems for adequate understanding of their mutual relations. 



The injection of blood vessels in the late (12 mm.) larva of 

 R. palustris in figure 40 is selected for the last of this series 

 as the best to illustrate the fate of the anterior segmental 

 veins, because the entire series of veins to which they belong 

 can still be distinctly seen, while the first three, which are 

 being especially studied, are shown in the same picture defi- 

 nitely incorporated in the great cutaneous and anterior verte- 

 bral venous systems. 



Thus the anterior vertebral vein is recognized in figure 40 

 as a definite venous path fixed in the early line of anastomoses 

 along the dorsal edges of the muscle segments. 



The connections from the neural plexus forming anterior 

 and posterior branches can be traced in the figure from the 

 extensive small-meshed plexus of the cord through numerous 

 small vessels, to the conspicuous dorsolateral vertebral trunk 

 which unites with the third intersegmental vein as in earlier 

 stages. In addition, the segmental veins of the lateral muscles 



