104 FRANKLIN P. REAGAN 



fording a possible clue to the origin of such erythrocytes; there 

 may be those who would be willing to consider these conditions 

 as evidence that cardiac endothelium can form erythrocytes. If 

 it had been possible to anticipate the unusual importance of this 

 embryo its history wotdd have been kept more fully. With a 

 number of others which had received like treatment this embryo 

 was segregated to a dish from which all embryos were removed 

 as soon as they were observed to develop a circulation. Later 

 it was removed to a dish in which embryos were placed only on 

 condition that their otherwise normal endocardia should be solid 

 at both extremities. Thus it is certain that this embryo once 

 possessed a complete endocardium which was solid at both ends 

 and dilated in the middle. On the eleventh day the upper end 

 of the heart was seen to display developing blood-cells which at 

 first were orange-colored. This color gradually deepened into 

 red by the fourteenth day, when the embryo was killed. Sections 

 of the heart reveal very interesting conditions. Figure 72 shows 

 a section through the distal (venous) end of the heart not far 

 from the point at which the endothelium is solid. The endo- 

 thelium is here of a cuboidal type similar to that in figure 71. 

 It stained purple. Inside the endothelium are a number of 

 erythrocytes which stained bright red. Outside the endothelial 

 cells lie the myocardial cells which stained pale blue. The plane 

 of section in figure 73 passes through a flexed portion of the 

 cardiac tube. Some of the erythrocytes in this section lie in a 

 column of erythrocytes which can be traced back to those in figure 

 72. It will be seen that the endocardium is interrupted. Some 

 of its cells are undoubtedly transforming into erythrocytes. At 

 the right of the lower extremity of the V-shaped remnant of the 

 endocardium on the left of the figure there are abundant transi- 

 tion stages between endothelium and erythrocytes. From the 

 main central mass of erythrocytes in this figure there appears in 

 section a rather sharp 'projection.' In this it is easy to follow 

 the contour of what probably was once the endocardium. The 

 cells at the apex of this projection stained a reddish purple. In 

 the middle of this figure it is seen also that some of the myocardial 

 cells have also undergone this transformation, especially in the 



