DEVELOPMENT OF THE MAMMALIAN SPLEEN 315 



very pronounced, but it is limited to the regions surrounding 

 the arteries. No indications were found of cells migrating toward 

 the follicles. On the other hand, migration from the follicles 

 into neighboring sinuses is quite frequently observed. This is 

 especially true where a large venous sinus extends parallel with 

 the lymphoid sheath of the artery. Here the small cells enter 

 the slow blood-current of the sinus and continue their differentia- 

 tion before being carried into the general circulation. Small lym- 

 phocytes were not observed in the pulp previous to the formation 

 of lymphoid arterial sheaths. The first small lymphocytes of the 

 follicles could, therefore, not be derived from the pulp. 



According to Schridde and Tiirk, lymphocytes are derived 

 from the endothelium of lymph-vessels and myeloid cells from 

 the walls of blood-vessels. The embryonic spleen is an ideal organ 

 for testing this theory. It has been shown that the mesenchyme 

 bordering the earliest sinuses is capable of giving off free cells, 

 most of which differentiate into erythrocytes. But it has also 

 been shown that this mesenchyme differs in no way from that in 

 other regions of the organ, and it can, therefore, not be regarded 

 as being more highly differentiated or specialized than is the 

 latter. The liberation of free cells is by no means confined to 

 the sinus mesenchyme, and hence the process has no value as 

 support for the theory of Schridde and Tiirk. Most of the free 

 lymphoid cells derived from the mesenchyme of all parts of the 

 organ, before the lymphoid arterial sheaths are established, dif- 

 ferentiate into erythrocytes. A few granulocytes also differenti- 

 ate from similar lymphoid cells, but the latter never show any 

 relationship to the endothelium of blood-vessels. The prepara- 

 tions show very clearly that the first cells to differentiate gran- 

 ules in their protoplasm are cut off from the mesenchyme, and 

 that they are usually located at some distance from the nearest 

 blood-vessels or sinuses. 



For the lymphocytes of the follicles the case is still clearer. 

 The follicles are expanded portions of a continuous lymphoid 

 sheath the cells of which are derived from the condensed mesen- 

 chyme surrounding the arteries. Lymphocytes of all types, but 

 especially the smaller ones, are liberated from this mesenchyme, 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 28, NO. 2 



