318 GEO. A. THIEL AND HAL DOWNEY 



concerned, there is no evidence whatever for the view that the 

 lymphoid cells of the two regions of the adult spleen are different. 



The question of the origin of the erythroblasts, myelocytes, 

 and megakaryocytes in the pulp of the embryonic and myeloid 

 metaplastic adult spleen still remains to be disposed of. Several 

 theories have been proposed by the dualists. They believe that 

 the cells in question may be differentiated in loco from myelopo- 

 tent leukoblastic pulp cells (Paremusoff) which are normally a 

 part of the pulp parenchyme, or that they are of extraparenchy- 

 matous origin, either from undifferentiated embryonic connec- 

 tive tissue or adventitial cells (H. Fischer), from cells of the vas- 

 cular endothelium (Schridde), or from the bone-marrow by way 

 of the blood-stream (Ziegler). 



According to the monophyletic view, the lymphocyte is not a 

 highly specialized cell. It is therefore capable of differentiating 

 into a granulocyte or erythroblast in response to definite changes 

 in environment brought about by the liberation of toxic irritants 

 in myeloleukemic conditions which, on account of the arrange- 

 ment of the circulation, affect the pulp first. With extensive and 

 prolonged metaplasia the peripheral regions of the follicles may 

 also become affected (Dominici, Weidenreich) . In the embry- 

 onic spleen unknown conditions favor the continued differentia- 

 tion of lymphocytes of the pulp (hemocytoblasts, large lympho- 

 cytes) along myeloid lines through late embryonic or postnatal 

 stages, while similar lymphocytes which are confined to the 

 territory of the follicles remain lymphoid. 



It is evident that exact knowledge of the history of the origin 

 and development of both pulp and follicles in the spleen must 

 bring the final decision in this question. 



According to Danchakoff, the follicular tissue of the bird spleen 

 is estabhshed by the condensation of the mesenchyme about the 

 arteries which grow into the organ at a time when it consists 

 entirely of 'pulp.' This mesenchymatous arterial sheath is 

 later transformed into a lymphoid sheath with foUicles by the 

 liberation of free cells from the mesenchymal syncytium. Dan- 

 chakoff states that the first free cells derived from the condensed 

 mesenchyme about the arteries are of the type of large hemocy to- 



