THE DURATION OF LIFE. 29 



explanation of the nature of heredity a problem the solution of 

 which may still occupy many generations of scientists. At present 

 we can hardly venture to propose any explanation of the real nature 

 of heredity. 



But the question must be answered as to whether the kind and 

 degree of reproductive power resides in the nature of the cell itself, 

 or in any way depends upon the quality of its nutriment. 



Virchow, in his ' Cellular Pathology,' has remarked that the cells 

 are not only nourished, but that they actively supply themselves 

 with food. If therefore the internal condition of the cell decides 

 whether it shall accept or reject the nutriment which is offered, it 

 becomes conceivable that all cells may possess the power of refusing 

 to absorb nutriment, and therefore of ceasing to undergo further 

 division. 



" Modern embryology affords us many proofs, in the segmentation 

 of the ovum, and in the subsequent developmental changes, that 

 the causes of the different forms of reproductive activity witnessed 

 in cells lie in the essential nature of the cells themselves. Why 

 does the segmentation of one half of certain eggs proceed twice as 

 rapidly as that of the other half? why do the cells of the ectoderm 

 divide so much more quickly than those of the endoderm ? Why 

 does not only the rate, but also the number of cells produced (so 

 far as we can follow them) always remain the same ? Why does 

 the multiplication of cells in every part of the blastoderm take 

 place with the exact amount of energy and rapidity necessary to 

 produce the various elevations, folds, invaginations, etc., in which 

 the different organs and tissues have their origin, and from which 

 finally the organism itself arises ? There can be no doubt that 

 the causes of all these phenomena lie within the cells them- 

 selves ; that in the ovum and the cells which are immediately 

 derived from it, there exists a tendency towards a certain determined 

 (I might almost say specific) mode and energy of cell-multiplica- 

 tion. And why should we regard this inherited tendency as con- 

 fined to the building up of the embryo ? why should it not also 

 exist in the young, and later in the mature animal ? The pheno- 

 mena of heredity which make their appearance even in old age 

 afford us proofs that a tendency towards a certain mode of cell- 

 multiplication continues to regulate the growth of the organism 

 during the whole of its life. 



