Franklin P. Mall. 235 
than the area vasculosa, we must admit that the histo-mechanical princi- 
ples justify us in assuming that, after the organ has ceased to grow, the 
rate and presence of the blood in all its capillaries are approximately 
equal. 
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Fic. 4. Part of the area vesicular of a chick incubated seventy-four hours. 
The dorsal aspect is presented. V, V, veins. The arteries are dark ae2is 
After Thoma. 
“In this organ, according to the first histo-mechanical principle, all 
blood channels in which the rate of fiow exceeds a certain maximum, must 
increase in lumen and become converted into arteries and veins. Vice 
