96 Heredity. 



tliis power of multiplication shall manifest itself. Most 

 of the organs of the body contain great numbers of cells 

 which are alike both in structure and function, and as it 

 is usually quite impossible to say how far the size of an 

 organ is truly hereditary, and how far it is determined 

 by surrounding conditions, it is, of course, imi)ossible to 

 say to what extent its mature structure is represented in 

 the ovum, but as we know that the size of most organs 

 varies, and may be increased or diminished by external 

 influences, we may be quite certain that the number of 

 independent cells w^hich make up the tissues and organs 

 of a mature organism, is very much greater than the 

 number represented by distinct particles in the ovum. 



It is not even necessary to suppose that all classes of 

 cells which are present in the adult are represented in 

 the ovum. In a mammal, for instance, certain epithelial 

 cells become converted into hairs, while others become 

 converted into glands or other specialized ej^ithelial 

 structures. 



It is not necessary to assume that all of these special- 

 izations are represented in the ovum, for we know' that 

 ordinary epithelial cells, in a part of the body where no 

 hair is normally developed, may, when inflamed, give rise 

 to hairs. It is therefore quite possible that each epithe- 

 lial cell may, when excited by the proper influence, tend 

 to become converted into a hair cell. Each cell of the 

 body may possess the tendency to manifest certain ju'op- 

 erties under certain conditions, and to manifest certain 

 other properties under other conditions, and the descend- 

 ants of a single c^ll may thus become modified in several 

 divergent directions, and each modification may be per- 

 fectly constant and characteristic of the race without 

 being hereditary; that is, without being represented in 

 the ovum by a particle with the same specialization. 



