166 CHARLES K. STOCKARD 



sider, the first and most important histo-mechanical principle 

 which determines the state of the hnnen of the vessel under 

 physiological and pathological conditions." 



Thoma again states this principle thus: ''In development 

 the vessels in which the blood stagnates degenerate, and in those 

 in which the rapidity is too great the lumen in enlarged." 



No one could fail to admire the splendid manner in which 

 Thoma attacked the problem of vasculogenesis in the yolk-sac 

 of the bird, or the ingenious way in which he attempted to 

 analyze the problem and deduce his three laws of histo-mechan- 

 ical processes. Yet the ' ' First and most important histo-mechan- 

 ical principle" does not apply to the development of vessels in 

 Fundulus embryos where there is no circulation of the blood. 

 Many vessels grow in size or "what is the same thing, show an in- 

 crease in the surface of the vessel walV without any "rate of the blood 

 current." The aorta in old embryos that never had their blood 

 to circulate and in which the heart is actually a solid string of 

 tissue, grows and attains a well developed lumen and a wall 

 lined with endothelium and surrounded by concentric fibers of 

 connective tissue as is shown in figure 49 in the previous paper, 

 drawn from such a specimen. This vessel is very slow to degener- 

 ate, in fact, it shows no sign of degeneration and actually persists 

 as long as the embryo is able to exist without a circulation, for 

 30 days or more. Vessels also develop upon the yolk-sac without 

 ever having a fluid to circulate through their lumen. Other 

 vessels are developed around the hlood cells of the intermediate cell 

 mass and the yolk-sac islands and in such vessels 'the blood stagnates' 

 from the first yet the vessels degenerate very slowly, in some cases 

 scarcely at all. 



In still other cases the blood may have circulated for a while 

 and then stopped for some time, but the vessels do not degenerate 

 as is proven by the fact that the circulation through them may 

 again be resumed. Such a sequence of events may be occasion- 

 ally observed in the experimental embryos. The function of the 

 vessel as a blood conductor, therefore, seems in these embryos of 

 Fundulus, both the early normal and those without a circulation 

 of the blood, to have little if anything to do with its early develop- 

 ment and not much effect on its ability to survive. 



