14 CHARLES R. STOCKARD 



pulling out of the heart may possibly be the cause of the failure 

 to develop its proper connections with the veins, or in some 

 cases to estabhsh and maintain its lumen. On account of these 

 mechanical deficiencies in the heart, we find that it is incapable 

 of propelling the body fluids and establishing the circulation of 

 the blood. The fluids thus accumulate in the large sinuses or 

 spaces, in most cases the coelomic spaces and in the case of Kupf- 

 fer's vesicle in an endodermic or endo-mesodermic cavity. 



The red blood cells become evident after the fluid accumulation 

 has partially taken place. They are always seen to originate 

 in definite localities and are never found in any place very dis- 

 tantly removed from these localized regions unless a partial 

 circulation or accident of some kind has occurred. Although 

 red blood cells in many places arise from wandering cells the 

 blood cells themselves have little capcity to wander. 



3. Early formation of blood cells in living embryos 



a. Intra-embryonic blood cells: The chief place of blood cell 

 formation is the intermediate cell mass which extends from 

 about the level of the anterior portion of the kidney back pos- 

 teriorly to behind the anus and well into the tail of the embryo. 

 This is the principal blood mass, but in addition to this, there are 

 present in all of these non-circulating individuals small blood 

 islands over the posterior and ventral yolk regions. These 

 blood islands are also present on the yolk of normal individuals 

 but in these the islands are swept away when the circulation 

 begins. 



b. Yolk-sac blood islands. A number of observations were 

 made on the embryos which failed to develop a circulation and 

 also on normal embryos to determine the significance and re- 

 lationships of the blood islands on the yolk as far as was possible 

 in the living individuals. 



In one experiment when the embryos were seventy-two hours 

 old, or just about the time that the heart-beat was beginning in 

 many, it was found that although the plasma was circulating 

 blood islands were present on the posterior yolk region. Ten 

 embryos were selected which showed these posterior yolk blood 



