EXPERIMENTAL METAPLASIA 353 



characters. With regard to the cytology of the changes involved 

 in regeneration and metaplasia, Kolliker ('85) considered that re- 

 generation of an organ or tissue cannot occur unless that organ or 

 tissue contains cells of an embryonic character, or at least contains 

 elements that are able to assume embryonic characters. The 

 question has been more fully dealt with by Adami ('00), who comes 

 to the conclusion that the fully differentiated cells of a tissue proper 

 never arise from cells that are themselves fully differentiated, 

 but during physiological regeneration arise from certain 'mother- 

 cells' which are normally present in the tissues. He considers 

 that ''under abnormal conditions, the fully differentiated func- 

 tioning cells of certain tissues are capable of proHferation and giv- 

 ing rise to cells of like nature, but this is only after a preliminary 

 reversion to a simpler, more embryonic type." 



It will be shown in the present paper that under the experimen- 

 tal conditions, the fibroblasts of Pecten revert to a somewhat un- 

 differentiated embryonic type, and then become converted into 

 columnar ciliated epithelium. 



DESCRIPTION OF TISSUES INVOLVED IN THE EXPERIMENTS 



An excellent and detailed account of the anatomy of Pecten 

 maximus and Pecten opercularis is given by Dakin ('09) in his 

 monograph on Pecten. The two species are very closely allied, 

 and except in point of size, resemble each other both anatomi- 

 cally and physiologically to a remarkable degree. 



The adductor muscle consists of two portions, bound together 

 by the same sheath of connective tissue, but differing in structure. 

 The larger, white and semi-transparent, consists of striated fibres. 

 The fibres of the smaller, which is of an opaque and dead white 

 appearance, and lies against the posterior surface of the larger 

 mass, are non-striated. It was into the larger mass that all 

 material in these experiments was introduced. vSuperficially 

 the muscle is covered with a layer of columnar epithelium con- 

 tinuous with that lining the mantle. 



There is a large blood supply to the muscle from the adductor 

 artery (Dakin), and it contains numerous lacunar spaces. Scat- 



