302 STUDIES IN GENERAL PHYSIOLOGY 
system, which is identical with the vascular system of the 
normal circulation in the points mentioned, to develop even 
in the absence of a circulation. The mechanical causes for 
the growth of the vessel-walls are, therefore, not to be sought 
inside the vascular lumen, but in all or in single cells of the 
vessel-wall. The giving off of branches is determined by 
internal causes acting within the cells of the vessel-wall, or 
through stimuli arising in their neighborhood which affect 
these walls as external stimuli affect the formation of stolons 
in Hydroids. It is possible, however, that the angles at 
which the branches arise from the main vessels do not corre- 
spond absolutely with the angles found in normal embryos. 
This point still remains to be investigated. Another ques- 
tion which I leave open is whether the circulatory system 
which is formed in the absence of a circulation is closed or 
not; that is, whether the capillary branchings of the arteries 
of the yolk pass over into the capillary branches of the 
veins. Mere morphological study speaks in favor of this 
idea, but to settle this point definitely further experiments 
must be made. I consider it as certain that the tiger-like 
marking of the yolk-sac of Fundulus is dependent upon 
the vascular system. 
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