364 B. F. KINGSBURY 



Hum. The lumen is lost and with it all surface relation for the 

 epithelium, which from this time grows loosely, forming the 

 cyto-reticulum or syncytium so characteristic of it, growth 

 occurring particularly and irregularly in the basal layers (next 

 the mesenchyme). The growth is further attended by an ac- 

 companying diffuse degeneration. In its relation to the under- 

 lying (surrounding) mesenchjaiie also the relations are atypical. 

 The growth co-ordination of these two tissue forms would typi- 

 cally determine the appearance of a (connective tissue) membrana 

 basahs. In the case in point, mesenchynal elements (so-called 

 large lymphocytes, primary wandering cells, leucoblasts) become 

 free and invade the cyto-reticulum, there prohferating rapidly, 

 forming the characteristic small cells with scanty cytoplasm (the 

 lymphocytes, small lymphocytes), the proliferation occurring 

 mainly in the peripheral growing zone — next the mesenchyme — 

 leading to the well known differentiation into cortex and medulla. 

 This peculiar relation of growth and proliferation continues, 

 in general coextensive with the presexual growth of the body, 

 then ceases and a more or less rapid involution of the structure 

 succeeds. Hassal's corpuscles are clearly phenomena of degener- 

 ation — cytoconglomerates — more marked and larger during the 

 period of involution. 



The thymus, from this point of view, may be regarded as an 

 expression of a persisfent and atypical growth of a non-adapta- 

 tive epithehum, this being accompanied by an altered epithelio- 

 mesenchymal relation whose characteristic feature is the infiltra- 

 tion of the epithelial (epithelioid) mass. The epithelial growth 

 is attended by an accompanying degeneration which becomes 

 more profound in the later period (involution). The thymus 

 transformation may thus be thought of as a reaction of degenera- 

 tion, prolonged and given its characteristic features by the 

 attendant growth. Such an interpretation at once raises a 

 number of important questions of marked biological and mor- 

 phological interest, both theoretical and practical, and for 

 the consideration of most of them the basis in fact is still insuffi- 

 cient. The far-reaching significance of the interpretation and 



