164 CHAELES R. STOCKARD 



the exact direction he considers his evidence to lead. He credits 

 Biitschli ('82) with having originated the coelom theory that 

 accords, so he thinks, with his evidence. This is entirely incor- 

 rect, at Butschli's theory is exactly on the other side. The 

 morphology of the yolk-sac of the chick, sheep and numerous 

 other animals, as the literature of the subject readily shows, 

 is entirely out of accord with such speculations. The yolk-sac 

 of the bony-fish shows this view to be really impossible and 

 there should be no longer any doubt that vessels arise from 

 loose and wandering mesenchymal cells in many animal species, 

 and certainly not from ingrowths from the coelomic epithelium 

 in any species. 



The formation of vessels on the yolk-sac of the teleost further 

 limits the generalization of the origin of larger vessels from 

 capillary nets. Thoma ('93) in his masterly study of the vas- 

 culogenesis of the yolk-sac of the bird, held that "the first vas- 

 cular spaces, the rudimentary capillaries, were formed by the 

 secretory activity of the cells forming their wall." These 

 capillaries formed an extensive net and the arteries and veins 

 arose secondarily and differentiated from the capillaries on 

 account of the flow of blood set in motion by the beat of the heart. 

 The anlage of the vascular system was the capillary. 



These principles of Thoma are not, however, applicable to the 

 development of vessels in the embryonic bony-fish. The aorta 

 arises as one or two vessels independent of any flow of blood or 

 the existence of a capillary net. The first vessels on the tele- 

 ostean yolk-sac are the large vitelline veins, as described by 

 Wenckebach, and the median vitelline vein or the net of vessels 

 in its place. These large important channels arise entirely inde- 

 pendently and separated from the capillary net if such exists 

 at the time. They also develop entirely independently of the 

 blood flow, and not as a result of the pressure due to the heart 

 beat. The capillaries and other vessels in many cases arise 

 separately or away from these primary vessels and finally con- 

 nect with them in a way similar to the connection formed be- 

 tween the Randvene and the venous end of the heart. Other 

 capillaries and small vessels arise as buds or sprouts from the 

 first formed veins on the yolk-sac. 



