52 CHARLES R. STOCKARD 



blood that the invertebrate animals, many of which possess 

 highly functionating white blood cells, amoebocytes, as well as 

 oxygen carrying corpuscles, are thought to derive these cells 

 and the vascular endothelium from mesenchyme and not from 

 endoderm. 



5. Has vascular endothelium a haematopoetic power? 



It has been mentioned in describing the origin of blood in va- 

 rious parts of these embryos that no observation could be inter- 

 preted to indicate that blood corpuscles ever arise from vascular 

 endothelium. The endothelium of vessels containing blood 

 never presents any cell in a transitional stage. These experi- 

 ments, I think, furnish a crucial answer to persistent claims 

 that vascular endothelium has the power to change into various 

 types of blood corpuscles. If vascular endothelium had such a 

 power, then one might expect that this power would show itself 

 in cases where it was most needed, for example, in these embryos 

 in which the blood has never circulated. The blood cells are 

 conf ned entirely to the intermediate cell mass and to the blood 

 islands on the yolk. 



The heart and aorta and numerous vessels in the head and 

 anterior portion of the body are lined with typical vascular 

 endothelium, yet in no instance has it been found that one of 

 these vessels contains a single red blood cell in any stage of 

 development. From these experiments, one is warranted in 

 making the bold assertion that the endothelial lining of the heart 

 and aorta is perfectly incapable of giving rise to any type of 

 blood cell. This fact has been mentioned in considering the 

 endothelium of the heart. When we now refer to figure 43, a 

 section through the anterior region of a four-day-old embryo 

 without a circulation, two dorsal aortae are shown. These 

 vessels are lined by typical embryonic endothelium but are 

 completely empty so far as cellular elements are concerned. This 

 is true of the dorsal aortae of all embryos from the earliest to 

 the latest stages when the circulation of the blood has been 

 prevented. Felix ('97) has also noted the fact that the aortae in 

 early normal Teleost embryos are invariably free of blood cells. 



