GENERAL CONSIDERATIONS 59 



side between the body of the chick embryo and the yolk-sac, 

 before vessels have appeared in the area pellucida, that blood 

 vessels make their appearance in the body of the embryo in a 

 typical manner on the operated side. These vessels differ from 

 those on the unoperated side only in size and rate of development, 

 differences which may be correlated with their reduced drain- 

 age area and the consequent diminished quantity of circulatory 

 fluid. 



These experiments of Hahn, and Miller and McWhorter have 

 conclusively shown that the yolk-sac angioblast cannot have 

 grown into the embryo on the operated side. In order to elimi- 

 nate the possibility, however, that the vessels on the operated side 

 may not have been formed in situ, but by an invasion of angio- 

 blast from the normal unoperated side, Reagan ('15) has recently 

 performed a set of experiments in my laboratory which con- 

 clusively disprove this contention. Instead of separating only 

 one side of the embryo from the yolk-sac, Reagan has been able 

 to develop the heads of chick embryos, which had been com- 

 pletely separated from the rest of the embryo and from the 

 yolk-sac, and in which endothelial-lined vascular channels of 

 mesenchymal origin were invariably found to appear. As in the 

 case of the experiments of Miller and McWhorter, the opera- 

 tions were performed at a time before it would have been pos- 

 sible for the intra-embryonic tissue to have been invaded by 

 yolk-sac angioblast. 



Graper ( ; 07), under the direction of C. Rabl, performed a set 

 of experiments on chick embryos, somewhat similar to those of 

 Hahn, and Miller and McWhorter, and, although he noted the 

 presence of independent blood-islands in the body of the embryo, 

 he was unable to interpret them as having been formed in situ. 



Jacques Loeb ('12) was the first to observe the effects produced 

 by certain chemicals' (NaCN) on the developing blood vessels 

 in fish embryos. He was able to produce a condition in which 

 a beating heart and blood were present, but no circulation; 

 a condition which, as stated by Schulte, can hardly be reconciled 

 with the doctrine that the vessels of the embryo have a primitive 



