ORIGIN OF BLOOD AND ENDOTHELIUM 239 



islands and isolated to determine whether the blood islands would 

 subsequently enter the circulation or what their fate would be. 

 Ten other embryos with a feeble heart-beat but with no circula- 

 tion yet also showing small posterior yolk-sac blood islands 

 were isolated for comparison. 



On the following day, nine out of ten individuals which had 

 estabHshed a vigorous circulation had no blood islands remaining 

 on the yolk. All of the islands had been taken into the circula- 

 tion or vascularized, so that instead of blood islands there was 

 now a network of vessels over that portion of the yolk and the 

 blood cells had entered the current. One of the ten embryos 

 exhibited an abnormal arrangement of the blood vessels that was 

 particularly instructive. On one side there was a large vein 

 running from the embryo out onto the yolk and on this side all 

 of the blood islands had disappeared forming a network of ves- 

 sels which conducted the blood to the venous end of the heart. 

 On the other side of the specimen there seemed to be a suppression 

 in the development of vessels near the embryo. The islands 

 were still in the same condition they had been on the previous 

 day except that the cells composing them had become much 

 redder so that there was now no doubt whatever that they 

 contained erythroblasts and early erythrocytes. 



The ten embryos which had no circulation on the previous day 

 were now found to present the following conditions: Two had 

 established a perfectly free circulation and all of the blood 

 islands had been swept away. In two other individuals cir- 

 culations were established in an abnormal manner so that many 

 blood islands still remained and presented a bright red appear- 

 ance. Six of the ten specimens had no circulation whatever 

 and the entire arrangement of blood islands over the yolk was 

 in exactl}^ the same condition as on the day previous, except that 

 the blood cells were now much redder in color. 



During the course of the experiments similar tests and obser- 

 vations have been repeated four times. In each instance isolated 

 groups of embryos showing yolk-sac blood island were selected 

 and examined in order to ascertain on the following day the 

 fate of these islands. In every case my experience was identi- 



