DEVELOPMENT OF THE MAMMALIAN SPLEEN 321 



Those believing in the 'predestination' theory can, of course, 

 use these facts in favor of their view. Then they must also as- 

 sume that the mesenchyme which enters into the formation of the 

 lymphoid arterial sheaths is 'predistined' to form both large and 

 small lymphocytes, while that of the pulp region can form only 

 large lymphocytes during the early stages. The fact that the 

 lymphocytes of the sheaths remain lymphoid, while the majority 

 of those in the pulp differentiate into erythrocytes, is then due 

 entirely to 'predestination.' 



Those who believe that the lymphatic and myeloid tissues are 

 absolutely separate and independent can also use the above facts 

 in support of their views. Because the 'lymphoid' tissue of the 

 aiterial sheaths and follicles appears later than the 'myeloid' 

 tissue of the pulp, its cells must be of different origin. 



This conclusion is not justified, how^ever, when all factors are 

 taken into consideration. It has been shown that the mesen- 

 chyme of the arterial sheaths is derived from that of the original 

 splenic rudiment and that it remains continuous with that of the 

 pulp portion of the organ. The development of certain vessels 

 into arteries causes, or is at least associated with, rapid prolifera- 

 tion of neighboring mesenchyme cells, resulting in marked con- 

 densation of the tissue surrounding the arteries (figs. 6, 8). This 

 mesenchyme then resembles that of the original splenic rudiment 

 before it contains many free cells. It soon assumes a looser 

 texture, owing to the liberation of many of its cells. The fixed 

 cells remaining form a supporting network which is directly con- 

 tinuous with and is derived from the mesenchyme of other parts 

 of the organ. 



Up to this point the history of the original rudiment is repeated 

 by the mesenchyme surrounding the arteries. The chief differ- 

 ence appears when it is seen that the continued production of 

 lymphoid cells in the regions of the future follicles does not lead 

 to differentiation of those cells along myeloid lines, as is the case 

 with the lymphocytes of the pulp. Since no foreign tissue has 

 entered the organ, it seems most logical to assume, with Weiden- 

 reich, Danchakoff, and others, that the arrangement of the vas- 

 cular system in the two parts of the organ is not only responsible 



