PLATE 18 



EXPLANATION OF FIGURES 



With the exception of the lymph heart, only blood vessels are injected in 

 figure 37, in a larva from the second period (fig. 2). The earliest stage of the 

 anterior vertebral vein is exposed by the absence of most of the superficial veins, 

 tributaries from the neuro-plexus and the adjacent segments uniting to form it in 

 its course to the third intersegmental vein. The first and second intersegmental 

 veins are also injected, although their connections are not complete. The first vein 

 receives chiefly tributaries from the anterior cutaneous system with facial connec- 

 tions. In a complete injection (fig. 38) of the late third period, the superficial 

 veins have been so greatly reinforced by contributions from the cutaneous artery 

 that the plexuses have overgrown and partly hide the primary veins of the anterior 

 segments. However, the interconnecting loops present no obstacle to tracing the 

 tributaries of the cutaneous system into the first two intersegmental veins and 

 the anterior vertebral from its three typical plexuses of origin into the third vein. 



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