198 W. B. CHAPMAN 



passed quickly to the embryo. That this large blood vessel was 

 formed from the capillary net by the functional activity of the 

 pulsating blister can not be doubted. On another occasion I 

 burned away all of an embryo anterior to the first somite. On 

 opening the egg again I found one of these small pulsating vesicles 

 which had developed from a remnant of the heart or the sinus 

 venosus and was pumping away after the manner described for 

 figure 17. I could mention a number of these freaks, but it 

 would have no bearing upon the problem. 



VII. DISCUSSION 



It appears, at first, that a number of conflicting factors have 

 been introduced into this experiment. Chief of which are the 

 following : 



1. The formation of the anterior vitelline vein after the time 

 when the circulation should begin. 



2. The failure of the large omphalo-mesenteric arteries and 

 veins and the posterior vitelline vein to appear. 



3. The pecuhar behavior of the sinus terminalis. 



4. The progressive peripheral growth and spread of the capil- 

 lary net, in combmaton with the central process of retrogression. 



That these factors were not due to accident, but to the enact- 

 ment of certain simple laws of development that have been long 

 recognized by certain leading scientists can readily be explained. 



Roux has divided the formation of the vascular system into 

 three stages, which are as follows : 



1. A stage of primary differentiation in which the formation 

 of the vessels is governed entirely by certain hereditary principles 

 due to inherited characteristics. 



2. A transitional stage between the first and third wherein the 

 hereditary formation is gradually supplanted by a process of 

 functional adaptation. 



3. A stage in which the further formation of all vessels is due 

 entirely to the mechanical forces acting through the circulation. 



The early formation of blood vessels which takes place before 

 the circulation begins is clearly not governed by the mechanical 

 forces acting through the circulation, and their formation can be 



