DEVELOPMENT OF THE MAMMALIAN SPLEEN 319 



blasts identical with those of the pulp portion of the organ. 

 Instead of differentiating into erythroblasts or granulocytes, as 

 do most of the hemocytoblasts of the pulp, these hemocytoblasts 

 proliferate rapidly and give rise to numerous daughter 'dwarf- 

 hemocytoblasts' which in turn differentiate into the small lympho- 

 cytes so characteristic of the follicular regions. 



According to Danchakoff, the differences in the differential 

 products of the two regions (small lymphocytes in the follicles, 

 erythroblasts and granulocytes in the pulp) are due entirely to 

 variations in the environmental conditions. The arterial sheaths 

 consist of very dense tissue in which the blood is confined 

 entirely to closed channels, while the pulp is of very loose structure 

 with a slow and 'open' circulation. The latter type of circula- 

 tion favors erythroblast and granulocyte differentiation, or it 

 at least permits any substance contained in the blood which 

 might influence cell differentiation to come in direct contact with 

 the cells of the pulp, while it could only reach those of the follicles 

 by slow diffusion. 



Such an explanation, while satisfactory to those of the mono- 

 phyletic school, would not satisfy the dualists, for they can 

 always claim that the ingrowing arteries bring 'adventitial' cells 

 with them which furnish the condensed tissue from which the 

 lymphocytes of the follicles are eventually derived. Herzog 

 claims that the endothelium of growing capillaries of the omen- 

 tum stimulated by implantation of foreign bodies gives rise to 

 numerous fixed and free adventitial cells from which numerous 

 small lymphocytes with dark nuclei are derived. 



While Danchakoff' s work seems to show clearly that the con- 

 densed tissue about the arteries is derived from the mesenchyme 

 of the original splenic rudiment, still the participation of the endo- 

 thelium of ingrowing arteries is not absolutely ruled out. 



Conditions are somewhat less complicated in the spleens of pig 

 embryos, for here the arteries do not grow into the organ at a 

 comparatively late stage as in the chick, but are developed from 

 small vessels which already exist in the primitive rudiment, i.e., 

 they are derived from vessels which already exist in the mesen- 

 chyme of the region in which the splenic rudiment is established. 



