16 HENRY MCELDEERY KNOWER 



of the first four segments through independent connections, 

 most of the outflow from in front being deflected around these 

 segments into the anterior cardinals (figs. 1 and 2). 



The segmented veins 



Examination of the venous drainage of the dorsal systems 

 of the trunk and tail in these early larvae reveals the same 

 general arrangement throughout the entire length, the out- 

 flow from the neural plexus of the cord running directly into 

 the intersegmental veins along the dorsal margins of the 

 lateral muscles. 



The neural plexus is thus immediately connected with the 

 adjacent middle zone or axial systems through both arteries 

 and veins, the dorsal segmental arteries giving off lateral 

 branches to the paraxial muscles in their passage to the cord, 

 while the veins from both cord and muscles drain outward 

 into the intersegmental veins. But while this simple arrange- 

 ment is preserved in the first four segments, as shown in 

 figure 1, the segments behind are brought into contrast by 

 the development of accessory connections between their veins, 

 which divert the blood from behind ventrally into the poste- 

 rior (P.C.) cardinal vein before it reaches the veins of the 

 first segments. 



The fourth intersegmental veins are the first of the series 

 to develop strong accessory loops extending dorsally from 

 their roots in the neural plexus. Similar loops extend into 

 the dorsal tail fin from the succeeding intersegmental veins 

 and over-arch the spinal cord anastomosing above it. They 

 thus furnish additional enlarged paths through the inter- 

 segmental veins, ventrally into the posterior cardinal veins, 

 to carry off blood coming forward from the rapidly developing 

 regions of body and tail. And since the posterior cardinals 

 run into the posterior angle of the pronephros, most of the 

 blood from the posterior regions of the body is here diverted 

 from passing through the anterior segments, which are located 

 immediately dorsal to the pronephros. 



Another series of anastomoses of enlarged ventral branches 

 of the intersegmental veins encourages additional diversion 



