366 



E. V. COWDEY 



(see Fig. 4). The space between the primary lobules is usually occupied 

 by loose connective tissue, containing vessels and nerves. In the case of 

 the secondary lobules the dividing trabeculse do not extend into the medulla 

 in man, though they often do so in lower forms. 



The Cortex The "Small Cells," their Origin and Nature. The cor- 

 tex differs from the medulla, in that it is densely packed with small cells, 

 indistinguishable morphologically from lymphocytes. There are two 

 views concerning their origin. According to the first, they are epithelial 



Fig. 4. Section of human thymus gland (magnification 36). 



in nature, arising through the frequent and rapid division of the epithelial 

 cells derived from the branchial pouches. Others, however, consider them 

 to be lymphocytes which have penetrated the organ and greatly changed 

 its appearance. Stohr has presented some points in favor of their epi- 

 thelial nature, and Bang claims that the thymus contains five times as 

 great a quantity of nucleinates as lymph glands, and concludes that the 

 cells in question are, therefore, not lymphocytes, Transitional forms 

 between the epithelial cells and lymphocytes, and the occurrence of lymph- 

 ocytes in the thymus before their appearance in other tissues, are also cited 

 as indicative of an epithelial origin. 



It is very difficult to say whether the cells differentiate in loco, or 

 migrate into the gland anlage from the surrounding mesenchyme. Con- 

 tradictory statements are to be attributed, in large measure, to the lack of 



