102 FRANKLIN P. REAGAN 



be overlooked. In photograph they stand out more clearly than 

 to the eye; this was accomplished by the use of color-screens. 



Let us now consider certain embryos devoid of circulation 

 which developed erythrocytes in their hearts. In these instances 

 the embryos were watched carefully at short intervals. It is 

 quite unlikely that a heart, partly solid, would hollow out be- 

 tween these intervals only to become solid again at such inter- 

 vals. If a blood-cell be found in the heart of an embryo in which 

 there had never been a circulation, there are at least the follow- 

 ing possibilities of its origin: The fully formed erythrocyte may 

 have reached that location (1) by its own independent power 

 of locomotion, (2) its mesenchymatous anlage might have reached 

 that location by active migration, (3) the erythrocyte in ques- 

 tion might be a transformation of an endocardial cell, (4) the 

 eiythrocyte in question might be a transformation of a cell that 

 would ordinarily have become a myocardial cell, (5) it might be 

 a transformed mesothelial cell. These seem to include most of 

 the possibilities. It is evident that if an investigator should 

 state that in the absence of circulation the heart "never con- 

 tains a single erythrocyte in any embryo at any age" he would 

 relinquish all right to claim that an erythrocyte found in that 

 location had reached it by any of the five possibilities just out- 

 lined. A claim that a blood cell never develops in a given loca- 

 tion excludes the possibility of claiming at the same time that a 

 blood-anlage had migrated into that location. 



The conditions in the heart of the embryo from which figure 

 49 was taken is of interest. No circulation could be observed in 

 this embryo. On the fifth day it was noted that both extremi- 

 ties of the heart were solid, though it appeared otherwise to be 

 normal. Previous to this time the embryo had received frequent 

 examination. The material seemed very favorable, owing to the 

 fact that its condition seemed unusually near the normal, consid- 

 ering the fact that there was no circulation. The pericardial 

 cavity was not greatly oedematous, and the heart was not 

 stretched. In fact it possessed a well-developed flexure. The 

 venous end of the heart was undoubtedly solid, and no yolk-sac 

 endothelium approached it. A means suggested itself of pre- 



