DEVELOPMENT OF VESSELS WITHOUT HEART 19V} 



accounted for only as a response to some hereditary principle 

 which as yet is not understood and which most scientists of the 

 present day are unwilling to believe exists. The early onset of 

 the circulation has been a factor which has obscured the cause 

 of the further development of the blood-vessels, and although it 

 was known that there was some formation of blood vessels such 

 as the heart and aorta before the establishment of a circulation, 

 it has only been since the publication of Miss Sabin's work on 

 the subject, that the extent of this early formation of blood-vessels 

 has been appreciated. 



It is now clear that by the time the circulation begins, the 

 embryo has a primitive but complete system of blood vessels. 

 With the advent of the circulation, which occurs about the thirty- 

 eighth hour of incubation, we have the beginning of the second 

 stage. In this stage the primary differentiation continues, but 

 is gradually replaced by the functional or mechanical factors due 

 to the heart-beat. In the normal chick, it is impossible to de- 

 termine just how much of the further formation is due to the 

 heart-beat, but in the operated chicks, the mechanical and func- 

 tional factors are eliminated and the further development is due 

 entirely to the continuance of the primary differentiation and 

 laying down of blood vessels independent of mechanical influ- 

 ences. That this continues after the time when the circulation 

 should begin is proved by the formation of the large anterior 

 vitelline vein by the fusion of the two, right and left, vitelline 

 veins. It is clear, therefore, that with the failure of the circula- 

 tion to start at the usual time, the further development of the 

 vascular system is not inhibited, but proceeds for a time in an 

 almost normal manner, that is, it goes a little further and this 

 relatively important embryonic vessel is laid down. 



The failure of the omphalo-mesenteric arteries to develop 

 proves that the formation of these vessels is due to mechanical 

 forces acting through the heart-beat, and in the absence of these 

 mechanical forces, due to the circulation, they are not evolved 

 from the capillary net. In view of the fact that I have never 

 at any time observed the slightest indication of the formation of 

 these vessels in my operated chicks, I am at a loss to understand 



