234 I'HE GERM-PLASM 



by Guignard to apply to plants also — is of decided importance 

 in connection with the conception that idants constitute the 

 hereditary substance. As the two nuclei are approaching one 

 another, their centrosomes become doubled, and the correspond- 

 ing pairs unite to form the two poles of a nuclear spindle (Fig. 

 1 8, III.), which direct the first cell-division leading to the for- 

 mation of the embryo ; this usually only occurs after the nuclear 

 membrane has cortipletely disappeared (Fig. i8, IV.). 



The process of fertilisation therefore consists in the union of 

 the nuclei of the two sexual cells within the maternal germ-cell, 

 and also of the bodies of the cells, together with their apparatus 

 for division. One half of the germ-plasm of the 'combination- 

 nucleus ' (' Copulationskern ") thus formed by the union of the 

 sexual nuclei consists of idants derived from the mother, and 

 the other half of those derived from the father, and the resulting 

 combination of two hereditary substances directs the ontogeny 

 and controls the building-up of the new individual. The entire 

 number of idants nevertheless always remains the same in all 

 the cells of the body : thus, for instance, if eight paternal and 

 eight maternal idants were brought together in the process of 

 amphimixis, there would be sixteen idants in every * cell in the 

 body of the individual arising from the fertilised ovum ; and if, 

 again, as represented in Fig. i8, there are only two idants in 

 each germ-cell, each somatic cell will contain four idants. 



The nature of sexual reproduction depends therefore on the 

 interminglino; of two hereditarv tendencies which are individuallv 

 different from one another ; or, to pass from the abstract to the 

 concrete, it depends on the union of two hereditary substances in 

 the first rudiment of the individual. We must next investigate 

 the manner in which this combination of hereditary substances 

 affects the composition of the germ-plasm. 



* A recent observation renders it doubtful whether ' every ' cell contains 

 the same number of idants ; but this need not here be taken into considera- 

 tion, as its importance cannot at present be estimated. 



