FERTILISATION 193 



together to form more complex units, named determinants, 

 which represent the separate parts of the organism. The de- 

 terminants are supposed to be aggregated together to com- 

 prise units of a still higher order, known as ids. These are 

 identified with the chroma tin granules. Every part of the 

 organism (or every character that it possesses) is believed to be 

 represented in an id. Moreover, Weismann assumes that the 

 ids vary slightly in related individuals, the differences in the ids 

 corresponding with the variations in the species. Lastly, the 

 ids are said to be arrayed in linear series so as to form idants. 

 Weismann identifies these with the chromosomes. It follows, 

 therefore, that each chromosome represents a particular group 

 of slightly differing germ-plasms. The purpose of variation, as 

 expressed in the terms of this theory, is to produce new com- 

 binations of heritable variations by the mixture of different 

 ids. And since the number of chromosomes, and consequently 

 the number of ids, is doubled as a result of the conjugating 

 process, the complexity of the chroma tin would become in- 

 definitely increased if there were no periodic reduction. But 

 this, according to Weismann, is provided against in the matura- 

 tion process of the gametes, when the quantity of chromatin 

 in the cells becomes reduced by one-half, as described in the 

 preceding chapters. 



The reduced number of chromosomes is supposed to contain 

 all the primary constituents of each of the two parents. And 

 what is more, according to this theory, every gamete con- 

 tains ids which are derived, not only 'from both the parents, 

 but also from the ancestors, all the immediate ancestors being 

 represented. 



Weismann's theory of the nature of fertilisation was ac- 

 cepted by many biologists as a working hypothesis, until the 

 disinterment of Mendel's discovery about ten years ago. The 

 confirmation of this discovery by numerous workers in different 

 fields has led to a revision of many of Weismann's conceptions. 



The original experiments of Mendel 1 were upon hybridisation 



1 Mendel, " Versuche iiber Pflanzen Hybriden," Verh. natur. f. Ver., in 

 Briinn, vol. iv., 1865. Reprinted in English in Mendel's Principles of 

 Heredity (Bateson), Cambridge, 1909. Mendel's work was rediscovered and 

 confirmed by de Vries, Correns, and Tchermak in 1900, and subsequently 



N 



