DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 165 



More recently Evans ('09) with a very efficient and delicate 

 method of injection has shown many of the larger intra-em- 

 bryonic vessels of the chick embryo to develop from a foregoing 

 capillary network. He found the same principles of develop- 

 ment that Thoma had observed on the yolk-sac to hold for the 

 development of certain vessels within the embryo. These prin- 

 ciples of vascular development Evans thought applied to verte- 

 brates generally, but such is certainly not the case, the large 

 vessels of the teleost embryo arise directly from associated 

 mesenchymal cells and are not preceded by a capillary net. 



His evidence was derived from injected vessels and could not 

 justify the statement, p. 512, of ''The presence always in the 

 embryo of a united vascular sj-stem" — "a single branched en- 

 dothelial tree." Such a united vascular system is rather late 

 in its establishment in the fish embryo and there is no "single 

 branched endothelial tree" present when the first blood vessels 

 are formed. These facts maj^ readily be demonstrated on the 

 living embryo by direct observation. 



The vessels of the yolk-sac and several of the larger vessels 

 within the body of the teleostean embryo form independently 

 of any foregoing capillary network. In the teleost, then, the 

 anlage of the vascular system is not the capillary hut the mesenchymal 

 cells which directly give rise to the chief arteries and veins, as well as 

 to numerous groups of isolated capillaries. Other small vessels 

 and capillaries grow as branches or sprouts from the arteries 

 and veins. 



Thoma advanced three laws for the formation and growth of 

 vessels. The first law was considered the most important, but 

 rather destructive evidence is thrown against it by the present 

 study. The law may be stated as follows: "The increase in 

 the size of the Imnen of the vessel, or what is the same thing, the 

 increase in the surface of the vessel wall, depends upon the rate 

 of the blood current." The vessel increases in size when the 

 rate is exceeded, becomes smaller when the rate is slowed, and 

 disappears lohen the flow is finally arrested. Thoma ('96) states: 

 "This law which brings the growth of the surface of the vessel 

 into dependence upon the rate of the flow of the blood is, I con- 



