324 GEO. A. THIEL AND HAL DOWNEY 



reticulum. In both cases the large lymphocytes are more or less 

 closely associated with the large reticular cells. Weidenreich 

 and Downey have shown that the explanation of this is to be 

 found in the fact that many of the large lymphocytes are cut off 

 directly from the reticulum in the same way that they are derived 

 from the mesenchyme of the splenic rudiment. 



This same hypertrophy and condensation of mesenchyme is 

 noted when the arterial sheaths are formed in the embryo, and it 

 is seen again when the folhcles are formed from the sheaths. The 

 condensed mesenchyme about the arteries is soon converted into 

 a lymphoid sheath by the process of isolation and Hberation of 

 many of its cells, which continues until the fixed tissue remaining 

 is small in quantity. A follicle originates within such a sheath 

 by the hypertrophy and prohferation of some of the fixed cells 

 within a limited area. This hypertrophied mesenchyme, or 

 reticulum as we may call it at this stage, then gives rise to nu- 

 merous large Ijaiiphocytes which by proliferation form numerous 

 small lymphocytes. It is only after the first follicles have been 

 formed that many large lymphocytes are encountered in the 

 lymphoid portion of the organ, for the early lymphoid sheaths 

 contain very few of them, as has already been pointed out (fig. 7). 



DEVELOPMENT OF FOLLICLES IN POST-NATAL ANIMALS 



The foUicles do not develop until after birth. As newborn pigs 

 were not available, the development of follicles was followed in a 

 series of one- to twenty-one-day old rabbits. In the one-day 

 rabbit some of the lymphoid arterial sheaths are similar to those 

 of the late pig embryos, i.e., they are composed of a mass of small 

 lymphocytes and a network of reticulum consisting of fine strands 

 and small cells. Large lymphocytes are rare. In other cases an 

 arterial sheath similar to the one described above is partially or 

 almost completely surrounded by a broad zone of hypertrophied 

 reticulum, the ceUs of which are very large and very numerous. 

 Numerous large and medium-sized lymphocytes are included in 

 this zone. Their cytoplasm shows varying degrees of basophilia, 

 and the structure of their nuclei varies from that which is typical 

 for the large lymphocyte to that of the nuclei of the reticular 



