Earliest Blood Vessels in Anterior I.inib Bud. 319 



irregular capillaries. He observed that the fortuitous position of 

 some of these capillaries with respect to the aortse and venous ostia 

 of the heart gave them a more constant and rapid circulation than 

 occurred in other capillaries of the mesh. Later stages showed these 

 capillaries became arteries and veins respectively. As the vitelline 

 vascular system grew, Thoma saw the same laws at work from center 

 to periphery, that the further elaboration of the arterial and venous 

 trees was the result of successive incorporation of adjoining por- 

 tions of the general capillary plexus. If these processes are at work 

 everywhere in the development of the vascular system, they furnish 

 us with a better understanding of angiogenesis, for the development 

 of a given artery or vein to any portion of the body cannot be due 

 to miraculous predestination but to the definite action of quite 

 definite physical laws. Capillaries first invade a region and the rela- 

 tion of these capillaries to the nearest arterial and venous channels 

 determines always the manner in which the new veins and arteries 

 shall arise. Then elaboration of arteries and veins is always the 

 result of hydrodynamical forces involved in the circulation. 



The application of Thoma's work to the development of the blood- 

 vessels in the body of the embryo has never been adequately tested. 



The method of injecting completely the embryonic vascular sys- 

 tem has furnished much evidence that the capillary plexus anlage 

 can be demonstrated for all the body's vessels. The 'preceding ac- 

 count of earliest circulatory conditions in the limb hud gains much 

 significance in this light, for before there can be said to be limb 

 arteries or veins, a primitive plexus of capillaries grows into the 

 limb tissue. From this plexus in later stages, arteries and veins 

 are formed. 



Rabl has shown the origin of several arteries in the fore-limb 

 region from capillary nets, but one must leave this interesting story, 

 the development of the later vessels, to another time. In the present 

 study, we have been able to see that in the limbs, the main vessels 

 themselves — the femoral and the ^:ubclavian arteries — exist originally 

 in the form of a capillary plexus. 



In conclusion I bog to speak with gratitude of the many sugges- 

 tions and helpful interest in the present investigation which I owe 

 to Professor Mall. 



