ORIGIN OF BLOOD AND ENDOTHELIUM 87 



the blood islands and may continue to arise from such a cell 

 during later development. 



Die ersten Endothelien und die ersten Blutzellen sind also beides 

 Mesoblast- resp. Mesenchymzellen. In den Blutinseln sehen wir 

 sie von unseren Augen aus einer gemeinsamen Quelle entstehen. Auch 

 in der spateren Entwicklung werden wir oft Gelegenheit haben, die enge 

 Verwandtschaft dieser beiden Arten von Mesenchymzellen zu beo- 

 bachten. 



This is merely a matter of. interpretation and not at all a 

 demonstrated fact. In reply to such a position we must call 

 for an explanation of the demonstrated fact presented on pre- 

 vious pages showing that vascular endothelium forms in a per- 

 fectly normal fashion within the heart and head region of em- 

 bryos without circulating blood, but in no case in early or late 

 stages was the endothelial lining of the aorta or other vessels 

 capable of giving rise to any type of blood corpuscles. Yet the 

 power to form blood corpuscles was abundantly present in the 

 same embryos as shown by the huge numbers of blood cells 

 within the blood forming regions, the intermediate cell mass 

 and yolk islands. Why do not the mesenchyme cells within 

 the liver and all vascular endothelium form blood when no cir- 

 culating blood reaches them? (If ever' there should then be 

 the stimulus to give rise to its formation). 



The red blood cell anlage is a definite mesenchyme cell or group 

 of cells and only members of this cell group possess the blood 

 forming power. To cite a parallel case, the liver cells are de- 

 rived from the common endodermal cell stock yet not all early 

 endodermal cells, in fact only a few, have the power to develop 

 into a liver, or a pancreas or a lung as the case may be. The 

 embryological argument is indeed rather loose that on account 

 of the fact of vascular endothelium and blood cells arising from 

 mesenchyme would assume, therefore, that these very different 

 cells had a common stem mother cell and later actually pos- 

 sessed some powers of transmutability. 



Maximow advances the interpretation that the first blood 

 cells in the area vasculosa are not all erythroblast or future red 

 blood corpuscles. These cells he designates as 'primitive blood 



