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 Fuiidulus is dependent upon 

 the vascular system. 



