ORIGIN OF BLOOD AND ENDOTHELIUM - 55 



blood cell formation tends to be towards or into the vessels and 

 the formation of white blood cells seems to be extra-vascular 

 or interstitial. 



It is recently claimed by Goodall ('07) that in the haema- 

 topoetic organs of the sheep embiyo such as the liver, there are 

 definite groups of prohferating cells forming the various types 

 of white blood corpuscles, and these are distinctly isolated from 

 other groups of proliferating erythroblasts. In the bone marrow 

 this same state of affairs has been described, and in a number 

 of diseased conditions of human marrow I have observed that 

 certain nests or groups of cells were gi\'ing rise to leucocytes 

 while other separate groups consisted of erythrocytes. This 

 observational e\ddence might seem to indicate that white and 

 red blood cells were arising from different parent cells. Yet in 

 normal embryos it is very difficult to obtain material which 

 will conclusively establish such a position, since both types of 

 cells are swept around by the circulation and are intimately in- 

 ter-mixed in all of the haematopoetic organs. 



It would seem that in these experimental specimens in which 

 the blood was prevented from circulating that there might 

 possibly be some way to distinguish completely the source of 

 origin of the white blood cells from the red blood corpuscles 

 if these sources were really different. Should the two types of 

 cells arise from the same common stem cell or parent cell, then 

 the white and red blood cells should be invariably found in associa- 

 tion in all embryos. If the two types of cells had different ori- 

 gins they might be found to occur in separate regions of the body 

 and the various sources could thus be readily differentiated. 



As frequently- stated, in the early intermediate cell mass and 

 among the cells immediately developmg out of this mass no 

 leucocytes or lymphocytes are found. The yolk-sac blood 

 islands also consist entirely of cells of the erythroblast type. 

 These observations are in accord with those of all other investi- 

 gators studying the development of the blood in the bony fish. 

 They have invariably described the intermediate cell mass 

 as being the source of red blood corpuscles and no one has ever 

 recorded either lymphocytes or leucocytes as arising from this 

 mass. 



