DEVELOPMENT OF THE THYMUS 441 



The latest modification of the pseudomorphosis theory which 

 is now generally accepted by investigators belonging to this 

 school, was set forth by Hammer in his investigations of the 

 thymus of human embryos ('05). According to him, the retic- 

 ulum of both the cortex and medulla and the Hassall's corpus- 

 cles are of epithelial origin. He then was in doubt as to the 

 origin of the lymphocytes in the thymus, but called attention to 

 the presence of 'wanderzellen' (lymphocytes) in the immediate 

 vicinity of the organ before and some time after they were pres- 

 ent in it. He also observed darkly stained cells in the thymus 

 of early developmental stages which, with only moderately high 

 magnification, could easily be mistaken for lymphocytes, but 

 with very high magnification could readily be recognized as 

 degenerating epithelial cells. He thus cast a doubt on the ori- 

 gin of lymphocytes from the epithelial cells of the thymus, as 

 is still held by some investigators, and pointed out as probable 

 an infiltration of the thymus with extrathymic lymphocytes 

 which have migrated into it from the surrounding mesenchyme. 

 This last view he was unable to prove conclusively on account of 

 a lack of sufficient range of developmental stages. His investi- 

 gations on numerous developmental stages of the Teleostean 

 thymus ('08) also led him to conclude that the fixed elements of the 

 thymus are of epithelial origin. He fully believed, however, 

 that the lymphocytes first present in the thymus of the Teleost 

 migrate into it from the mesenchyme and there, through repeated 

 division, give rise to the numerous lymphocytes found in it 

 in its fully developed condition. In his latest work ('11) on 

 the development of the hmnan thymus he makes no mention of 

 a migration of lymphocytes into the thymus from the surround- 

 ing mesenchyme. 



Maximow in his work ('09 b) on the developing thymus of 

 mammals (rabbit, guinea-pig, cat, rat and mouse) and on the 

 thymus of the Axolotol ('12) also holds that the fixed elements 

 of the thymus are of epithelial origin, while the lymphocytes 

 first present in that organ have migrated into it from the mesen- 

 chyme and, through repeated division, form the numerous 

 small lymphocytes of the thjmius in later developmental stages. 



