DEVELOPMENT OF THE MAMMALIAN SPLEEN 317 



As shown by the writers and others, the splenic rudiment soon 

 becomes converted into a loose, spongy mesenchymatous organ 

 with many wide blood sinuses, and numerous free cells held in 

 place by the strands of fixed cells remaining from the original 

 mesenchyme. At this time, the great majority of the free cells 

 are either erythroblasts or hemocytoblasts which will differen- 

 tiate into erythroblasts at a later period. There are no follicles 

 or arteries with lymphoid sheaths at this stage. The structure of 

 the entire organ corresponds to that of the pulp of late embryonic 

 or adult spleens, excepting that in the adult spleen erythropoiesis 

 is nearly or completely suppressed. The frequently expressed 

 statement that the early embryonic spleen consists entirely of 

 'pulp' seems, therefore, to be essentially correct. 



Because the so-called lymphoid tissue of the spleen, i.e., the 

 tissue of the lymphoid sheaths of the arteries and of the malpigh- 

 ian bodies or follicles, appears later in the development of the 

 organ, and because during myeloid metaplasia in the adult the 

 erythroblasts and myelocytes appear almost exclusively in the 

 pulp, it has been assumed that the follicles and lymphoid sheaths 

 consist of tissue which is foreign to the original spleen in its early 

 embryonic stages. 



Even the lymphoid cells of the two regions are supposedly 

 different in origin and function; those of the folhcles being true 

 'lymph-adenoid' lymphocytes and those of the pulp histogenous, 

 myelopotent, myeloblastic, or leukoblastic lymphoid cells, ordi- 

 narily remaining dormant and undifferentiated, but differentiat- 

 ing into myelocytes or erythroblasts when under the influence of 

 the proper irritant, or those of the pulp belong to a special race of 

 'splenocytes' (Naegeli, Tlirk, Banti) which are exclusively sple- 

 noid and not included in either the myeloid or lymphoid tissues.^ 



Dominici, Downey, and Weidenreich and others have shown, 

 however, that every type -of lymphoid cell found in the pulp can 

 be duplicated in the follicles, and they have also brought suf- 

 ficient evidence to show abundant emigration of true lympho- 

 cytes from follicles to pulp. In so far as cell structure alone is 



^ For literature on this question see the papers by Downey and Weidenreich, 

 Hertz, Werzberg, H. Fischer, Ziegler, and Klein. 



