THEORY OF THE GERM-PLASM 429 



As a non- nucleated fragment of egg fertilised by a sperm will 

 in some cases e.g. sea-urchins develop into a normal larva, as 

 an unfertilised ovum e.g. of sea-urchin may under certain 

 treatment develop into a normal larva, it is obvious that each of 

 the germ-cells has in its nucleus a complete set of hereditary 

 qualities. 



As a single egg often produces two complete organisms (true 

 twins), and in some cases e.g. the parasitic Hymenopteron 

 Encyrtus produces a legion of embryos, it is obvious that, 

 however the hereditary qualities are contained in their 

 chromatin vehicle, they can be very readily and rapidly multi- 

 plicated by division ; and every one is aware how many germ- 

 cells can be produced in a short time by a sexually mature 

 animal. 



It is now well known for a large number of animals and plants 

 that during the maturation of ovum and spermatozoon the 

 number of chromosomes is reduced to half the normal number 

 characteristic of the body-cells of the species, so that the union 

 of sperm-cell and egg-cell results, not in a doubling of the usual 

 number of chromosomes (as would be the case were there no re- 

 duction), but in a restoration of the normal number. It there- 

 fore follows that a reduction of the number of chromosomes by 

 a half does not in any way affect the completeness of the heritage. 

 " The halved hereditary substance still contains the whole mass 

 of primary constituents." 



By following up this line of argument, Weismann was led to the 

 theoretical conclusion that each of the chromosomes must con- 

 tain a complete equipment of hereditary constituents, and that 

 the germ-plasm represented by all the chromosomes in the 

 germ-cell must include several " complexes of primary con- 

 stituents," each complex sufficient in itself to form a complete 

 individual. In other words, the fertilised egg-cell as a mosaic 

 of " ancestral plasms." 



" I call the idioplasm of the germ-cells Germ-plasm, or the 



