326 GEO. A. THIEL AND HAL DOWNEY 



Not all of the large Ij^mphocytes of the follicles and lymphoid 

 arterial sheaths of the young rabbits are derived from the fixed 

 tissue. Some of them are derived from small lymphocytes which 

 enlarge and gradually assume the characters of large lymphocytes 

 as they pass from the central to the peripheral regions of the fol- 

 licles or sheaths. 



This transformation of small to large lymphocytes takes place 

 at the same time that other large and medium-sized lymphocytes 

 are being cut off from the fixed tissue. Special activity on the 

 part of the fixed tissue is, therefore, associated not only with the 

 liberation of Ijrmphocytes from the reticulum, but also with the 

 transformation of small to large lymphocytes. 



The derivation of lymphocytes from the fixed tissue in postnatal 

 animals is a well-known process which has been discussed in 

 detail by Weidenreich and Downey, by Tschaschin, and by Mol- 

 lier ('13). It proves that the reticulum of the lymphoid organs, 

 the fibroblastic tissue of the omentum and possibly also that of 

 the loose connective tissue retains the capacity of the embryonic 

 mesenchyme to liberate free lymphoid cells. 



The germ centers of the follicles of spleen and lymph nodes 

 are regions in which the reticulum is especially active, as is mani- 

 fest by the hypertrophy, increase in number, isolation, and 

 transformation of its cells. This special activity on the part of 

 the reticulum results in the formation of lymphocytes from it, 

 and it is also associated in some way with the proliferation of 

 the Ij^mphocytes surrounding the germ center. According to 

 Sabin and others, the development of the follicle is preceded by 

 a special arrangement of the vascular capillary network. It is 

 possible that the hypertrophy of the reticulum in the follicles of 

 the postnatal animals is due to the formation of a dense capillary 

 network which is absent in the lymphoid sheaths of the embryo. 

 However, the proof for this is lacking. 



