DEVELOPMENT OF THE MAMMALIAN SPLEEN 325 



cells of this zone. On the other hand, the cytoplasm of many 

 of the reticular cells is more basophilic than is usually the case 

 and their nuclei contain more than the usual amount of chroma- 

 tin. Such cells are more or less isolated from the general syncy- 

 tium and some of them are entirely free. There are numerous 

 transition stages between these cells and typical large and me- 

 dium-sized lymphocytes. In other words, it is not difficult to 

 trace the large and medium-sized lymphocytes directly to the 

 fixed cells of the reticulum. 



In some cases a more or less circumscribed area of hypertro- 

 phied reticulum occurs in the marginal zone of the lymphoid 

 sheath. The reticular cells of these regions are large, protoplas- 

 mic, rounded cells whose cytoplasm is of varying degrees of baso- 

 philia. Among them, but more often surrounding them, are 

 large and medium-sized lymphocytes, many of them in mitosis. 

 The nuclear and cytoplasmic characters of some of these cells 

 indicate that they have been cut off from the reticulum. Small 

 lymphocytes are numerous in the peripheral regions of these 

 areas and there are numerous transitions between them and the 

 larger cells. 



It is evident that this entire group of cells represents an early 

 follicle with a germ center composed of large reticular cells from 

 which some of the lymphocytes have been derived. This is the 

 typical structure of the follicles of the adult rabbit at one stage 

 of their activity. They are developed in the marginal zone of 

 the lymphoid sheath at some distance from the artery, and many 

 of them remain in this position in the adult. The lymphoid 

 arterial sheaths remain prominent in the rabbit, and the follicles 

 are merely enlarged or condensed portions of the sheaths, usually 

 associated with hypertrophied reticulum. 



The appearance of large lymphocytes in the lymphoid portion 

 of the spleen of postnatal animals is associated with marked 

 activity on the part of the reticulum characterized morphologic- 

 ally by hypertrophy and increased basophilia of its cells. In the 

 embryo the condensed and hypertrophied mesenchyme about the 

 arteries gives rise to small lymphocytes only, while in the pulp 

 of the embryo the first lymphoid cells cut off from the condensed 

 mesenchyme have the general characters of large lymphocytes. 



