SUMMARY' AND CONCLUSION 453 



indepejide7itly variable, must be represented in the germ-plasm 

 by a special determinany Were this not the case, the cell in 

 question could only vary in common with other cells which are 

 controlled by the same determinant. 



The germ-cell of a species must contain as many determinants 

 as the organism has cells or groups of cells which are inde- 

 pendently variable from the germ onrvards, and these de- 

 terminants must have a definite mutual arrangement in the 

 germ-plasm, and must therefore constitute a definitely limited 

 aggregate, or higher vital unit, the - id.' ,j 



J From the facts of sexual reproduction and heredity we must 

 conclude that the germ-plasm contains many ids, and not a 

 single one only. The formation of hybrids proves that the two 

 parents together transmit all their specific characters, so that in 

 the process of fertilisation each contributes a hereditary sub- 

 stance which contains the primary constituents of all parts of 

 the organism, — that is, all the detenninants required for build- 

 ing up a new individual. The hereditary substance becomes 

 halved at the final stage of development of the germ-cells, and 

 consequently all the determinants must previously have been 

 grouped into at least two ids^ But it is very probable that 

 many more ids are usually present, and that in many cases their 

 number far exceeds a hundred. 



It cannot be stated with certainty which portions of the ele- 

 ments of the germ-plasm observable in the nucleus of the ovum 

 correspond to ids, though it is probable that only parts of, and 

 not the entire •chromosomes,'' are to be regarded as such. 

 Until this point can be definitely decided, our further detailed 

 deductions will be based on the view that the nuclear rods 

 (chromosomes) are aggregates of ids, which we speak of as 

 ' idants.' In a certain sense, the latter are also vital units, for 

 they grow and multiply by division ; and the combination of ids 

 contained in them, although not a permanent one, persists for 

 some time. 



The ' germ -plasm,'' or hereditary substance of the Metazoa 

 and Metaphyta, therefore, consists of a larger or smaller number 

 of idants, which in turn are composed of ids ; each id has 

 a definite and special architecture, as it is composed of de- 

 terminants, each of wliich plays a perfectly definite part in 

 development. 



The development of the primary constituents, contained 



