282 VERA DANCHAKOFF 



versa, could be expected only in the case if the structural con- 

 ditions of the pulpa be changed and corresponded to those in 

 which the small lymphocytes normally develop. 



The study of the normal development of the spleen in the 

 chick allows the following conclusions : 



1. The anlage of the spleen develops from the mesenchyme of 

 mesodermal origin. It appears at the end of the 4th day of 

 incubation. It consists first of a loose mesenchymal tissue, 

 which gradually becomes denser and finally is transformed into 

 a syncytium. 



2. The development of a veinous vascularization transforms 

 the spleen anlage into a homogeneous pulpa-like tissue. This 

 stage is characterized bj^ a well developed granulopoiesis and a 

 potential erythropoiesis. The stem cell for both directions of 

 differentiation is the lymphoid hemocytoblast, which develops 

 at the expense of mesenchymal cells. The line of the stemcell 

 differentiation depends upon the conditions to which the stem- 

 cells are submitted. 



3. The development of the follicles and of the lymphoid tissue 

 coincides with the development of the arterial vascularization. 

 The small lymphocytes develop at the expense of the lymphoid 

 hemocytoblasts and their differentiation is preceded by appearance 

 of generations of small-sized hemocytoblasts. The small lymph- 

 ocytes in the spleen, as well as in the thymus, appear in regions, 

 characterized by special structural conditions of the environment. 



Different kinds of cells appear, as final results of differentiation 

 processes, observed in the spleen. Reticular tissue cells, and 

 endothelial cells, erythrocytes, granulocytes, small lymphocytes 

 and macrophages develop in the same organ. Can anything 

 definite be told on the basis of the study of normal spleen devel- 

 opment about the origin of these cells? Are all the cells of the 

 mesenchymal spleen anlage identical and polyvalent, their further 

 development being determined by the structural environmental 

 conditions, or though apparently identical in morphological 

 structure their potencies to different development are predeter- 

 mined by intrinsic imperceptible differences between them? 



The general outlines of the development of the spleen anlage 



