METAPLASIA AND NEOPLASIA 273 



METAPLASIA 



A few words, however, must be said with regard to metaplasia 

 in general, for the law of reversion to a more embryonic type is 

 distinctly in evidence throughout, and indeed helps us to under- 

 stand how, for example, a columnar-celled epithelium can be 

 converted into a squamous-celled epithelium, and one form of 

 connective tissue into another ; or, otherwise, metaplasia is 

 never direct, but is only brought about by preliminary reversion 

 to a more embryonic type, or, where mother cells are present, 

 by the modified development of cells derived from the mother 

 cells, the influence of environment altering the character of those 

 daughter cells during the period of growth. 



NEOPLASIA 



If the principle be correct that the more highly differentiated 

 a cell, the less its capacity for proliferation ; or, in other words, 

 the more the daughter cells depart from the type of the mother 

 cells, the less their proliferative capacity, then the converse 

 would seem to hold that the more the daughter cells in a given 

 tissue retain the characteristics of the mother cells, the greater their 

 proliferative capacity. Or to continue the argument if through 

 any cause the daughter cells in a tissue do not attain full specific 

 differentiation, then they are peculiarly liable to proliferate and 

 function, not as specific cells, but as mother cells. And further, 

 as Bizzozero has pointed out, tissues in which the cellular elements 

 exhibit frequent mitosis are those which more especially are liable 

 to be the seat of excessive growth and tumour formation. 



I am far from saying that this is the one principle concerned 

 in the development of the blastomata, or tumours proper, but 

 I would urge that it is one important principle. It is outside 

 the limits of this article to consider what is the essential stimulus, 

 or what are the stimuli, leading to neoplastic growth ; accepting, 

 however, for the moment, that, whether temporarily or per- 

 manently, some stimulus or stimuli be in action, at least we 

 gain a comprehension of why in the first place histoid tumours 

 (in which one or other tissue is more or less faithfully reproduced) 

 are essentially benign ; and in the second place why malignancy 

 and excessive cell proliferation, which is the essence of malig- 



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