EXPERIMENTAL METAPLASIA 351 



epithelium of the larynx into a squamous type as the result of 

 chronic irritation, the conversion of the pavement epithelium of 

 the bladder into the columnar type found in papillomatous over- 

 growths, and a similar change in cases of ectopia vesicae where 

 even glandular crypts of a very simple type may appear. 



Of special interest are the experiments of Wolff ('93) on the 

 eye of the larval newt and salamander. In these animals, if the 

 lens be removed, it is regenerated from the iris, and it has also 

 been shown that the retina itself can, under certain conditions, 

 produce bodies resembling lenses. Here we have a case of meta- 

 plasia between totally dissimilar tissues, differing greatly in their 

 mode of formation, but which still are both of epiblastic origin. 

 A somewhat similar case is that described by Saxer ('04) who found 

 a tissue resembling columnar epithelium lining the cysts that are 

 of relatively common occurrence in gliomata. 



Among the mesoblastic tissues there are many common in- 

 stances of metaplasia. For example, the ossification of tendons 

 and muscles, the formation of true bone in the lungs, in old pleural 

 and pericardial adhesions, and in the fibroid valve of chronic 

 endocarditis. Harvey ('07) has experimentally caused the forma- 

 tion of bone in the walls of arteries of rabbits by injury, and a 

 similar formation is often found in cases of arterio-sclerosis of 

 long standing. 



Examples of the change of tissue derived from one germ layer 

 into closely allied tissues derived from the same germ layer might 

 be multiplied indefinitely, but it must be noted that there are 

 many pathologists who d9ny this frequent occurrence of meta- 

 plasia, and explain these changes in other ways. Foremost of 

 these is Ribbert. He considers, for example, that the replace- 

 ment of columnar by squamous epithelium is due to the over- 

 growth of included islands of squamous cells, under altered con- 

 ditions of environment, and he has other explanations for most of 

 the cases here cited. His conclusions, as summarized by Adami 

 ('08) are to the effect that ''only tissues that, while externally 

 different, possess nevertheless the same histogenetic capacities, 

 can undergo metaplasia, one into the other." On the other hand, 

 Leo Loeb ('99) has recorded that in cases of epithelial regeneration 



