18 HENRY MCELDERRY KNOWER 



entering at this angle the posterior cardinal vein continues 

 obliquely and ventralward to meet the anterior cardinal vein 

 which runs along the perpendicular front side straight down- 

 ward from the ear. The two cardinals join at the inferior 

 angle of the pronephros back of the heart to form the duct of 

 Cuvier, which then enters the heart on either side. Both 

 cardinal veins receive branches from around the tubules of 

 the gland, and also contributions from the first three inter- 

 segmental veins above (fig. 1). 



The first lymphatics 



The entire complex of pronephric gland and veins of the 

 anterior segments is thus seen to lie dor sally across the open- 

 ing of the duct of Cuvier (fig. 1), a location in which the first 

 lymphatics have been found in the embryos of most forms 

 studied (Sabin, '13; Hoyer, '34; Grodzinski, '27). 



The arterial component of the pronephros, the glomerulus, 

 lies just medial to the gland, dependent from ventrolateral 

 branches of the aorta, the efferents in this stage joining the 

 vitelline plexus on the oesophagus. They connect with the 

 venous sinus later, through the cardinals. In addition to the 

 vascular conditions discussed above (fig. 1), important genetic 

 relations between the primitive kidneys and the first lympha- 

 tics were pointed out by Knower in 1908 and 1913-1914. 



In the report of 1913-1914, the statement was made that 

 "the first lymph vessels in the frog form a small and super- 

 ficial dorsolateral plexus, which drains into the pronephric 

 sinus through a short vein. On this plexus, in the frog, the 

 anterior lymph heart soon appears and facilitates the drain- 

 age into the venous channels surrounding the pronephric 

 tubules." Now, after further study of many more injections, 

 the same statement must stand. 



The conditions pictured in figure 1 are typical of the first 

 lymphatic stage, and the early relations of the gland to the 

 lymphatics associated with the veins surrounding the tubules 

 is emphasized. 



