ORIGIN OF BLOOD AND ENDOTHELIUM 61 



Many points mentioned in the previous pages indicate that 

 blood cells change their mode of behavior as the conditions of 

 the embryonic vessels and body are changed during development. 



A careful consideration of various embryos as well as the 

 different regions of the same embryo suggests that erythroblasts 

 only multiply in spaces unlined by endothelium, whether these 

 be on the yolk-sac, or within the embryonic liver, spleen or bone 

 marrow. The unlined spaces thus afford an environment in 

 which for physical or chemical reasons the erythroblasts are 

 able to multiply and reproduce themselves. When, however, 

 such spaces or channels become converted into endothelial 

 lined tubes then the erythroblasts tend to differentiate into 

 erythrocytes and very soon cease to reproduce themselves any 

 further in this location. In the intermediate cell mass of the 

 fish embryo for instance, one notices in early stages many divid- 

 ing erythroblasts. Just about the time that this mass becomes 

 completely enclosed by vascular endothelium, this division pro- 

 cess slows down and finally ceases although the confined erythro- 

 cytes may never be able to leave the vessel. Soon after being 

 surrounded by vascular endothelium the erythrocytes assume 

 a passive non-productive state and remain in this condition 

 throughout their existence. 



One of the earliest places of blood cell proliferation or haema- 

 topoesis is the sinuses on the yolk-sac. Almost as soon, however, 

 as these sinuses become converted into the yolk vessels, blood 

 cell production ceases in the yolk-sac and only blood circulation 

 takes place. 



The haematopoetic processes are then transferred to the em- 

 bryonic liver and this in most vertebrate embryos is an important 

 seat of blood cell multiplication. The multiplying erythroblasts 

 are never enclosed by endothelium. In other words, they are 

 not within the vessels but their final products are invariably 

 budded or divided off into the sinusoids. Finally, the spaces 

 in which blood cells multiply in the liver become obliterated or 

 converted into endothelial lined vessels and spaces and very soon 

 after this takes place the haematopoetic processes cease in this 

 organ. The liver cells themselves or the interstitial tissues of 



