DEVELOPMENT OF THE MAMMALIAN SPLEEN 323 



portions of the organ. The myeloid reaction of the pulp in the 

 embryo and during myeloid metaplasia of the adult is the result 

 of factors which do not usually exist in the normal adult. In the 

 spleen of the latter, excepting in those animals in which myelo- 

 cytes can be found in the spleen pulp throughout life, no types of 

 cells are found in the pulp which are not also seen in the follicles, 

 as was shown especially by Dominici and by Weidenreich and 

 Downey. 



In their figure 10 Downey and Weidenreich have shown that in 

 postnatal animals lymphocytes, both large and small, which are 

 identical with those of the normal pulp and follicles may differ- 

 entiate granules in their cytoplasm. That the typical small 

 lymphocyte can differentiate granules is of special importance, 

 since it shows that the genuine, fully differentiated lymphocyte 

 (Ehrlich) is capable of differentiating into a granulocyte. It is 

 therefore, quite unnecessary to assume that the myeloid reaction 

 depends on the existence of a special line of 'micromyeloblasts' 

 or ^microlymphoidocytes' (Pappenheim) , or 'myeloblasts' of 

 Schridde and Naegeli of either local or foreign origin. 



The development of the follicles or malpighian bodies from the 

 arterial sheaths of the spleen is of special interest. Downey and 

 Weidenreich have shown that in adult animals rapid increase in 

 the number of lymphocytes of the follicles of both lymph nodes 

 and spleen is associated with hypertrophy of the reticulum of the 

 follicle. The hypertrophied reticulum consists of large, pale 

 cells which contain few or no fibers in their cytoplasm. Usually 

 this hypertrophied reticulum forms a pale mass in the center 

 of the follicle, the so-caUed 'germ-center.' The accumulation of 

 lymphocytes around this nucleus of large reticular cells forms the 

 largest part of the follicle. 



In some animals the arrangement of cells is frequently just the 

 reverse of what has just been described. The hypertrophied 

 reticulum forms a peripheral ring about the follicle and the 

 accumulation of lymphocytes is in the center. In such cases the 

 large lymphocytes are also near the periphery, while in the usual 

 type of follicle the majority of them are to be found in a zone 

 immediately surrounding the central nucleus of hypertrophied 



