ORIGIN OF BLOOD AND ENDOTHELIUM 97 



Vascular endothelium in the fish embryo has no haemato- 

 poetic function. 



9. Neither lymphocytes nor leucocytes have been found to 

 arise in the yolk-sac blood islands nor within the intermediate 

 cell mass. 



The embryonic white blood cells are most abundant in the 

 anterior body and head regions, and these cells occupy extra- 

 vascular positions usually lying among the mesenchymal cells. 



The sources of origin of the white and red blood corpuscles in 

 Fundulus embryos are distinct, and these two different types of 

 cells cannot be considered to have a monophyletic origin except in 

 so far as both arise from mesenchymal cells. 



The adult blood of Fundulus contains lymphocytes and several 

 varieties of granular leucocytes. 



10. There is evidence to indicate that definite environmental 

 conditions are necessary for blood cell proliferation or multi- 

 plication. Blood cells do not normally divide when completely 

 enclosed by vascular endothelium. This is the key to the 

 shifting series of so-called haematopoetic organs found during 

 embryonic development. 



Erythroblasts lying about spaces unenclosed by vascular 

 endothelium proliferate steadily and give off their products 

 into the spaces from which they find their way into the embryonic 

 vessels. Should such an erythroblast be carried by the circu- 

 lation to another unlined space it may become arrested' there and 

 again undergo a series of divisions giving rise to other erythro- 

 blasts. When, however, these spaces become lined by endo- 

 thelium the blood cell reproduction stops. 



It most embryos the earliest blood cell formation occurs in 

 the yolk-sac blood islands. The cells in these islands continue 

 to divide until they become surrounded by endothelium, then 

 the yolk-sac blood islands lose their haematopoetic function and 

 become a vascular net through which the blood circulates. The 

 liver now takes up the r61e of harboring dividing blood cells 

 within its tissue spaces, when these spaces becdme vascularized 

 by endothelium, here again the blood cells no longer multiply 

 but merely circulate. 



