90 FRANKLIN P. REAGAN 



All the mesenchjrme cells on the yolk which later formed endo- 

 thelium and blood-cells and chromatophores migrated ven- 

 trally and forward beyond a zone comprising the posterior yolk- 

 surface. Thus it is seen that the posterior surface of the yolk is 

 free of endothelium, blood-cells, and of chromatophores. Sec- 

 tions of this embryo show that no endothelial tubes exist in this 

 posterior yolk-region. There is no connection of yolk-sac ves- 

 sels with those of the embryo's body posteriorly, anteriorly or 

 laterally. The yolk-sac vessels represent a large number of dis- 

 continuous spaces with intimately associated chromatophores. 

 In the tail region of living embryos it was possible to follow free 

 mesenchyme cells in the formation of endothelium, though the 

 process was here complicated by the presence of large numbers of 

 mesenchyme cells. These results on teleost endothelium con- 

 firm those of Wenckebach, Raffeale, and Stockard. 



If allowed to estimate the relative values of mechanical ex- 

 perimentation on chick embryos and chemical treatment or hy- 

 bridization of teleost embryos, I should say that the former is 

 the more conclusive. In the body of the teleost, it is impossible 

 to say whether discontinuous spaces have always been out of 

 continuity. It is true that there has been observation (66, p. 

 586) indicating that a vessel once having formed, has little or 

 no tendency to collapse. I was formerly led to believe this to 

 be true. Studies during a subsequent spawning season of Fun- 

 dulus have convinced me that this question of collapsing ves- 

 sels will bear further investigation. In regard to the j'^olk-sac 

 vessels of the teleost, it may be stated that no one has doubted 

 the local formation of yolk-sac vessels in any group of verte- 

 brates. 



PART II. THE ORIGIN OF BLOOD CELLS IN TELEOST 



EMBRYOS . 



As many writers have observed, the actual genesis of the blood 

 is obscured by the fact that blood cells are swept from their 

 places of origin before their differentiation is completed. By 

 the motion of body fluids, blood-cells become swept into places 

 where they did not originate. As Stockard was perhaps first to 



