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THE POPULAR SCIENCE MONTHLY. 



restriction may conceivably follow from a limitation in the number of cell-genera- 

 tions possible for tbe cells of eacb organ and tissue" (p. 28). 



"Tbe above-mentioned considerations show us that the degree of reproductive 

 activity present in the tissues is regulated by internal causes while the natural 

 death of an organism is the termination — the hereditary limitation — of the process 

 of cell-division, whioh began in the segmentation of the ovum " (p. 30). 



Now though, in the above extracts there is mention of " inter- 

 nal causes " determining " the degree of reproductive activity " of 

 tissue cells, and though, on page 28, the " causes of the loss " of the 

 power of unlimited cell-production " must be sought outside the 

 organism, that is to say, in the external conditions of life " ; yet 

 the doctrine is that somatic cells have become constitutionally 

 unfitted for continued cell-multiplication. 



" The somatic cells have lost this power to a gradually increasing extent, so 

 that at length they became restricted to a fixed, though perhaps very large, num- 

 ber of cell-generations " (p. 28). 



Examination will soon disclose good reasons for denying this 

 inherent restriction. We will look at the various causes which 

 affect their multiplication and usually put a stop to increase after 

 a certain point is reached. 



There is first the amount of vital capital given by the parent ; 

 partly in the shape of a more or less developed structure, and 

 partly in the shape of bequeathed nutriment. Where this vital 

 capital is small, and the young creature, forthwith obliged to 

 carry on physiological business for itself, has to expend effort in 

 obtaining materials for daily consumption as well as for growth, 

 a rigid restraint is put on that cell-multiplication required for a 

 large size. Clearly the young elephant, starting with a big and 

 well-organized body, and supplied gratis with milk during early 

 stages of growth, can begin physiological business on his own 

 account on a great scale ; and by its large transactions his system 

 is enabled to supply nutriment to its multiplying somatic cells 

 until they have formed a vast aggregate — an aggregate such as 

 it is impossible for a young mouse to reach, obliged as it is to 

 begin physiological business in a small way. Then there is the 

 character of the food in respect of its digestibility and its nutri- 

 tiveness. Here, that which the creature takes in requires much 

 grinding-up, or, when duly prepared, contains but a small amount 

 of available matter in comparison with the matter that has to be 

 thrown away ; while there, the prey seized is almost pure nutri- 

 ment, and requires but little trituration. Hence, in some cases, an 

 unprofitable physiological business, and in other cases a profitable 

 one ; resulting in small or large supplies to the multiplying 

 somatic cells. Further, there has to be noted the grade of vis- 

 ceral development, which, if low, yields only crude nutriment 

 slowly distributed, but which, if high, serves by its good appli- 



