300 GEO. A. THIEL AND HAL DOWNEY 



tion with the mesenchymal syncytium. When free, these cells 

 imdergo further changes in structure and staining reaction until 

 they become the large lymphocytes which are typical of all 

 hematopoietic organs. 



From the explanation that has been given of the early system 

 of vascularization it could perhaps be said that ideal conditions 

 are present in the splenic tissue for the development of a new 

 hematopoietic organ by metastasis of a preformed blood-forming 

 organ, the cellular products of which might be held in the meshes 

 of the splenic syncytium where they could find favorable condi- 

 tions for multiplication and differentiation. This interpretation 

 was held by most of the early hematologists, and is still being 

 maintained by some members of the dualistic school. Stockard 

 is of the opinion that the blood-forming organs are obliged to 

 shift to different positions in the embryo in order that spaces 

 unlined by an endothelium may be utilized for blood-cell prolifer- 

 ation. He states that "There is evidence to indicate that defi- 

 nite environmental conditions are necessary for blood-cell pro- 

 liferation and multiplication. Blood-cells do not normally divide 

 when completely enclosed by vascular endothelium. This is 

 the key to the shifting series of so-called hematopoietic organs 

 found during embryonic development." He further emphasizes 

 the fact that the 3ells in the yolk sac blood-islands "continue to 

 divide until they become surrounded by endothehum." They 

 then ' ' lose their hematopoietic function and become a vascular 

 net through which the blood circulates. The liver now takes up 

 the role of harboring dividing blood-cells \vithin its tissue spaces, 

 when these spaces become vascularized by endothelium, here 

 again the blood-cells no longer multiply but merely circulate." 



Such an interpretation denies the possibility of local formation 

 of blood elements from the mesenchymal cells of the blood- 

 forming organs, and also from the endothelial walls of the early 

 blood-vessels. Many authors, however, regard the endothelium 

 as a definite source of blood-cells of the various classes. Accord- 

 ing to Schridde, cells of the myeloid series are derived exclusively 

 from the endothelium of the blood-vessels, while Maximow, 

 Danchakoff, and Jordan derive all types of blood-cells from the 



