28 HENRY MCELDEERY KNOWER 



through them from the dorsal segmental arteries is steadily 

 augmented with the continued development of the dorsal 

 structures. 



Thus, in figure 2, a new line of venous loops has appeared, 

 connecting the four anterior veins at their junctions with the 

 cord plexus. These loops overhang the lymph heart plexus 

 in the tissues of the body wall dorsal to the pronephros, and 

 lie along the dorsal margins of the lateral muscle segments. 

 They are, on the one hand, extensions of the earlier connec- 

 tions between the intersegmental veins and the plexuses of 

 the hind-brain and cord ; and on the other, new tributary veins 

 initiating drainage of the body wall of this region. 



The two adjacent plexuses, once established, continue to 

 grow independently in frog larvae, the lymphatics spreading 

 from the primary plexus of the anterior lymph heart, while 

 the three first intersegmental veins persist in their original 

 relations, to become parts of important definitive venous sys- 

 tems, the anterior vertebral and great cutaneous veins. 



The differentiation of the anterior vertebral and great 

 cutaneous veins from the primary plexuses of the anterior 

 intersegmental veins will be described at the end of this paper, 

 with figures 37 to 40 (pp. 78-82). 



PRIMARY DISTRIBUTION OF LYMPHATICS (SECOND PERIOD) 



The arrangement of the primary group of lymphatics de- 

 scribed for figures 3 and 4 is found to be essentially the same 

 in other injections of this period, although pronounced varia- 

 tions in special minor features occur in connection with in- 

 creased activity in the spreading of the system. Thus the 

 parent plexus (lymph heart plexus) in figures 5 and 6, as in 

 figure 4, differs in no important character from that in figure 

 3, although the latter is more compact, less defined, and has 

 fewer outgrowths. (The absence of anterior outgrowths in 

 fig. 3 is apparently due to lack of development in an early 

 stage, not to a failure of injection, which is exceptionally 

 strong throughout.) 



