286 VERA DANCHAKOFF 



current from the growing graft. However, the systematical study 

 of the gradual enlargement of the spleen urged to finally admit a 

 growth of the embryonic spleen tissue in loco. This local growth, 

 at least in the beginning, seemed to be caused by an intense uni- 

 form stimulation of the mesenchymal cells and of the lymphoid 

 hemocytoblasts and their further differentiation. After the 

 hypertrophic character of the enlargement of the spleen had been 

 established, it was proposed on the basis of this fact to test 

 the validity of the monogenetic conception of the blood origin. 

 If true, the monophyletic interpretation implied analogous 

 changes in other hematopoietic organs as seen in the spleen. 

 The chief character of the changes observed in an embryo, after 

 grafting of an adult spleen, consists indeed in a stimulation of the 

 whole hematopoietic tissue. The hematopoiesis, stmiulated ex- 

 perimentally follows strictly the fundamental principles, estab- 

 Hshed for birds and reptiles during embryonic and adult life. The 

 hematopoiesis develops however in a peculiar way; first in that 

 groups of cells, which normally would slumber indefinitely, 

 become involved in the process; secondly, that an outburst of 

 hematopoiesis is incited at a time when the normal hematopoiesis 

 is conveyed in definite channels; and finally, that various direc- 

 tions of differentiation are displayed by hemocytoblasts in places 

 where they are usually absent. The study of the changes in the 

 spleen after grafting will form a basis for a comparative study 

 of the changes, incited by the same intervention in other hem- 

 atopoietic organs. 



Since the stimulation leads to different changes according to 

 the stage at which it has been applied, the results of the stimu- 

 lation in early and later stages will be described separately. 

 As mentioned, grafts of hematopoietic tissue on the chick al- 

 lantois take easily, if grafted from the end of the 6th day of 

 incubation and on. The grafts, made before the establishment 

 of arterial vascularization in the spleen, incite in the spleen 

 anlage uniform proliferation, which may considerably vary in 

 intensity. The changes are different if the stimulation has 

 been applied after the arterial vascularization has taken place. 



