432 HEREDITY AND DEVELOPMENT 



fidence in the conclusion that the nuclear material is the physical 

 basis of the heritable qualities. 



7. Microscopic vivisection has shown that the continued vitality 

 of a cell-fragment depends on whether that fragment has any 

 nucleoplasm or not. 



Ancestral Plasms. — Assuming that the chromatin substance 

 of the nucleus of the germ-cell is the vehicle of the inheritance, 

 Weismann argued that it " contains not only the primary con- 

 stituents of a single individual of the species, but also those 

 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. Gradually 

 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 accumula- 

 tion 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 conclusion that the 

 material basis of inheritance is a mosaic of ancestral plasms. 



