428 HEREDITY AND DEVELOPMENT 



of several, often even of many, individuals." In fact it is 

 a mosaic of " ancestral plasms." But what evidence is there 

 of this ? 



A fertilised egg develops into an organism by cell-division. 

 For a time it is demonstrable that the nucleus of each of the 

 daughter-cells into which the fertilised egg-cell divides contains 

 paternal and maternal chromosomes in equal number. Gradu- 

 ally differentiation sets in, and various kinds of body-cells with 

 specialised structure and function appear ; but often it is quite 

 demonstrable that the maternal and paternal contributions are 

 forming the warp and woof of the organism. While most of the 

 ever-increasing crowd of embryonic cells undergo differentiation, 

 some do not, but remain unspecialised, retaining the characters 

 of the fertilised ovum. From this lineage of unspecialised cells, 

 as we have explained in Chapter II., the germ-cells of the new 

 organism arise. By-and-by when the organism becomes mature, 

 these germ-cells are liberated, and each of them will have, by 

 hypothesis, chromosomes derived from the original father and 

 mother. But fertilisation will occur between these liberated 

 germ-cells and others whose chromosomes are likewise derived 

 from another father and mother, assuming that the usual 

 cross-fertilisation occurs. Thus there comes to be an ac- 

 cumulation of contributions from different ancestors, though 

 the actual number of visible stainable bodies or chromosomes is 

 always kept the same. It seems impossible to evade the con- 

 clusion that the material basis of inheritance is a mosaic of ancestral 

 plasms. 



It is interesting to recall Darwin's memorable saying : " Each 

 living creature must be looked at as a microcosm a little 

 universe, formed of a host of self-propagating organisms, in- 

 conceivably minute and as numerous as the stars in heaven." 

 He thought of his hypothetical gemmules as including not 

 merely the contributions of the immediate parents, but ancestral 

 items from even remote progenitors. 



