82 ELIOT R. CLARK 



not true of blood-vessel, as it is of other tissues, that two such 

 phases exist, the vascular system would furnish a marked excep- 

 tion — at least, so far as our present knowledge of other tissues 

 and organs goes. The question as to how far the vascular sys- 

 tem might develop without heart-beat has not yet been satis- 

 factorily worked out and presented. 



SUMMARY AND DISCUSSION 



The results of these studies on living blood-vessels are: 



a. An extensive vascular development takes place, in the 

 early embryonic stages, which is independent of the mechanical 

 factors concerned with the circulation of the blood and the in- 

 terchange of substances through the endothelial wall. During 

 this stage, which to some extent precedes the inauguration of 

 cardiac pulsation, it has been found that the aorta develops 

 beyond the capillary stage (Thoma ('93) ) and that a number 

 of other main arteries and veins are formed (Miss Sabin, '17) 

 while several observers, by producing embryos with the heart 

 beat eliminated, have found, apparently, that a number of the 

 other main vessels develop. My own studies on frog embryos 

 without hearts, show that extension of the blood-vascular system 

 during this primary stage takes place by sprouting, and by the 

 formation of anastomoses between sprouts. 



Thus the vascular system, like other systems about which 

 we have knowledge, differentiates and is carried a considerable 

 distance on its developmental course — manifesting the property 

 of extension by sprout formation, and forming some of the larger 

 arteries and veins — as a result of 'hereditary inertia,' or 'self 

 development.' 



This stage, however, comes to an end relatively early; and 

 the vascular system, for its further development into the compli- 

 cated and nicely balanced system of the adult animal comes to 

 be dependent upon the mechanical factors concerned with the 

 pull and push of outside tissues, with blood-pressure and blood- 

 circulation, and with the interchange of substances through the 

 wall. The picture presented in the tails of a-cardiac tad-poles 



