ORIGIN OF VASCULAR TISSUES 105 



Upper part of the figure. In figure 74 which is still more proxi- 

 mal is shown a region in which both endocardium and myocar- 

 dium have completely transformed into erythrocytes; the en- 

 tire content of the mesothelial covering consists of strongly eosin- 

 ophilous cells of a typical erythrocyte character. It is a region 

 in which endothelium has been observed in the living condition 

 in this embryo. 



3. Hematopoesis in the liver 



Stockard believes himself to have demonstrated that liver 

 tissue is incapable of forming blood cells, and that it has no 

 hematopoetic function in the bony fishes. His conclusions are 

 based on the fact that he failed to find blood cells in the livers 

 of chemically treated embryos which developed without a cir- 

 culation. If this conclusion necessarily follows, one might claim 

 that the actual finding of erythrocytes in the liver of an embryo 

 which had not had a circulation would prove the liver to be a 

 blood-producer. 



One question which should be settled is this: are embryos de- 

 void of circulation capable of developing livers to a stage in 

 which their potentialities are given a fair test? Even though the 

 chemically treated embryo may retain its life for a time equal 

 to that in which the normal liver appears to produce blood-cells, 

 it is questionable whether the conditions in the chemically 

 treated embryo can ever approach the normal condition. 



If a blood cell should develop locally in the liver it might be 

 of mesodermal or of entodermal origin. The vertebrate liver is 

 morphologically a growth into the ventral mesentery; conse- 

 quently it has a covering of mesothelium and of mesenchyme. 

 The liver of Fundulus is a growth of the gut into the mesen- 

 chyme which in turn is covered by mesothelium. These meso- 

 dermal tissues are properly considered a portion of the liver tis- 

 sue. If any of the mesodermal elements- of the liver should turn 

 into blood cells, such transformations must necessarily be reck- 

 oned with in the study of the liver. It is conceivable that hem- 



