368 B. F. KINGSBURY 



function of the fibroblasts which now produce fibrous tissue in large 

 quantity. . . . The activity of lymphocytes and of connective 

 tissue is to some extent independent of each other; while in many 

 cases the two act together, in some cashes the one prevails, in other 

 cases the other (p. 727). . . . Connective tissue and lympho- 

 cytes maA' therefore be regarded as organs of attack, lying quiescent 

 under ordinary conditions, but exerting their efforts as soon as within 

 given limits certain pathologic changes take place (p. 728). 



Might it not well be that lymphocyte infiltration and con- 

 nective tissue invasion represent two sides of a fundamental 

 mesenchymal reaction? 



Finally, I desire to call attention to Metchnikoff's conception 

 of senescence (''In senile atrophy there is always present the 

 atrophy of the higher and specific cells of a tissue and their 

 replacement by hypertrophied connective tissue") and to his 

 accompanying well-known view of the frequent destruction in 

 old age of epithelia (as in the kidney) bj^ phagocytes — leucocytes 

 from the blood. Without stopping to comment on his recogni- 

 tion of two 'distinct' reactions, the connections that the inter- 

 pretation here discussed has with pathology is sufficiently obvious. 



It is also recognized that the interpretation of the thymus 

 involves the biological and moi-phological interpretation of the 

 lymphatic nodes (including hemolymphatic nodes) as well as 

 the simpler or more complex accumulations of lymphatic tissue 

 in mucous membranes and glands. From this aspect the lym- 

 phatic nodes have scarcely been considered as yet. 



Furthermore, the thymus problem is a part of the larger prob- 

 lem of the blood cells and hematopoiesis in general. The 

 unitary character of the group of the blood cells, while not es- 

 tablished beyond controversy, has been made highly probable 

 through the work of Maximow and of Weidenreich particularly. 

 It is customary to express it in terms of cell lineage, and to desig- 

 nate it as the 'monophyletic' mode of origin of the blood cells. 

 It is a conception compatible with the acceptance of either 

 the derivation of the primary blood cells from the entoderm, 

 in accordance with the angioblast theory of His, or a derivation 

 from and development out of the mesoderm, although the evi- 

 dence, as the writer sees it, mainly supports the latter inter- 



