106 FRANKLIN P. REAGAN 



atopoesis in the liver might also consist in a direct transforma- 

 tion of entodermal cells into blood-cells. 



There seems to be abundant evidence that mesenchyme cells 

 caught up in the liver tissue are capable, just as is true of other 

 mesenchyme cells, of turning into erythrocytes. This alone 

 does not render the liver a 'hematopoetic organ.' Applied to 

 any structure which embryos devoid of circulation are capable 

 of developing, the term 'hematopoetic organ' is probably a mis- 

 nomer. Figure 81 shows a section of the liver diverticulum. A 

 part of the tissue in this section lies near the junction of the di- 

 verticulum with the gut. The erythrocytes which developed in 

 the periphery of this liver may have come from mesenchyme 

 cells captured by the outpushing entodermal tissue. This fig- 

 ure is from the embryo of which figures 62, 63, 64 and 75 repre- 

 sent sections. Reasons have already been given for the belief 

 that this embiyo never developed a circulation. The erythro- 

 cytes in figure 81 are neither surrounded by endothelium nor 

 dangerously near any endothelium. It is impossible again to 

 illustrate adequately the differentiations in these tissues by the 

 use of one color. The erythrocytes are brilliant red, the liver 

 tissue is grayish purple, and the mesenchyme is pale blue. With- 

 in the liver are certain large cells more coarsely granular than 

 the surrounding cells; their nuclei are larger than those of the 

 surrounding liver-cells, and somewhat eosinophilous. The nu- 

 clei of the adjacent liver cells are dense bluish black. The cyto- 

 plasm of these larger cells has assumed a purplish pink color. 

 Some of these cells, or at least some of the cells located near them, 

 approach rather closely the erythrocyte characteristics. In some 

 sections of this liver it is impossible to tell whether the erythro- 

 cytes present have developed from cells similar to these, or from 

 mesenchymal inclusions. 



Figure 80 shows a section through the liver of the embryo of 

 Erimyzon from which figures 76 to 79 were also taken. Lying 

 promiscuously in the liver tissue are differentiating erythrocytes 

 which are out of relation to all endothelium. It seems improb- 

 able that the partly solid heart of this embryo could have caused 



