INTRODUCTION I T 



the body of the cell but in its nucleus, and several discoveries 

 were made shortly afterwards which rendered it certain that the 

 idioplasm is to be looked for in the ' chromosomes ' of the nucleus, 

 — those rod-like, coiled, or grain-like structures which are dis- 

 tinguished by their remarkable affinity for certain colouring mat- 

 ters. I shall return to the proof of this fact in the following 

 section. 



From this time onwards each subsequent theory of heredity 

 was based on a firm foundation of fact. It was now not only 

 known that the phenomena of heredity among the higher organ- 

 isms are connected with a definite substance, but the seat of the 

 latter had also been ascertained. I now therefore adopted this 

 firm basis for my theory of the germ-plasm, if I may call the im- 

 perfect form in which it then existed by such a name : I localised 

 the germ-plasm in the nuclear substance of the germ-cell, and 

 supposed that ontogeny was due to a qualitative change in it, 

 which hands the idioplasm on from one generation to the next 

 bv means of nuclear- and cell-division. But I soon went further. 

 From the fact of sexual reproduction, which brings together 

 equal amounts of paternal and maternal germ-plasm at each fer- 

 tilisation, I inferred not only the composition of the germ-plasm 

 out of a number of units, th^ '■ ancestral germ-plasms'' ('Ahnen- 

 plasmen '), but also the necessity of a reduction of the germ- 

 plasm each time to one-half of its bulk, as well as a reduction of 

 the number of the ancestral germ-plasms contained in it.* The 

 hypothesis of the ' reducing divisions of the germ-cells ' has been 

 thoroughly substantiated by subsequent observations : in fact it 

 has even been proved that in many cases this reduction occurs 

 exactly as I had foretold and had represented in a diagrammatic 

 figure ; f that is to say, by the non-occurrence of the longitudinal 

 division of the chromosomes which occurs in the ordinary nuclear 

 division, and by the distribution of these in the daughter-nuclei. 

 This holds good for the ovum as well as for the sperm-cell in 

 animals, and, as far as is known, in plants also. The germ-cell 

 must in all cases by division get rid of half of its nuclear rods, — 

 that is to say, of its germ-plasm, — in order to become capable 

 of fertilisation. This fact supports the other assumption of the 

 construction of the germ-plasm from ancestral germ-plasms, 

 which are not minute vital particles — analogous to Spencer's 



* ' On the Number of Polar Bodies,' &c., 1887. t Ibid. 



