LYMPHATICS IN FROG LARVAE 17 



of blood into the posterior cardinals (back of the sixth seg- 

 ment, fig. 1) and besides, a conspicuous new detour is soon 

 developed laterally. 



This is the lateral vein, which at first merely a line of 

 plexiform connections between the series of intersegmental 

 veins (fig. 1), in the stage shown in figure 2 has become a 

 strongly defined lateral trunk draining venous tributaries 

 from dorsally, laterally and ventrally into the cardinals back 

 of the sixth segment and enters the cardinals through a series 

 of openings as far back as the tenth segment. In later stages 

 the posterior segment of this vein terminates at the tenth 

 segment, and is known as the posterior vertebral vein. 



The development of the lateral vein appears to be evidently 

 a reaction of adjustment for maintaining the balance of blood 

 pressures from front to back and dorsal to ventral. We find 

 no anterior continuation of this lateral vein in this period 

 through the first segments as has been claimed, the anterior 

 vertebral vein which terminates in the third intersegment 

 being a later, independent formation. 



The blood from the dorsal systems having been followed 

 into the posterior cardinal veins, these veins can be traced 

 in figures 1 and 2 forward along the ventral borders of the 

 lateral muscles. They are accompanied on either side by 

 small lateral cardinal veins which receive some of the inter- 

 segmental veins. There are connecting vessels uniting the 

 medial with the lateral cardinals, as well as with the aorta. 

 The main current, however, is still continued through the 

 original medial cardinal vein (sub-cardinal) on either side. 



Ventral to the fourth segment, the cardinal vein enters the 

 posterior angle of the pronephros, which has been located as 

 a rounded body behind the branchial region of the head lateral 

 to the anterior end of the yolk mass, below the first four 

 muscle segments. In side view (figs. 1 and 2) the pronephros 

 appears as a right triangle, with the base directed dorsally 

 against the muscles and the right angle in front, the apex 

 projecting down back of the heart. The posterior angle of 

 the gland lies in its base below the fourth segment. After 



