246 THE GERM-PLASM 



In Ascan's jncgalocephala the number of idants is only two or 

 four ; but as far as we know, a greater number is present in the 

 case of all other animals, and also in that of plants : thus there 

 may be eight, sixteen, thirty-two, and even a hundred or more.* 

 A simple and single reduction, such as we have hitherto assumed, 

 will therefore in general secure a very considerable amount of 

 variety as regards the combinations of primary constituents 

 caused by the reducing division. Nature seems, however, to 

 have aimed at a far greater degree of variety, at any rate in the 

 case of animals, in which a double instead of a single reduction 

 of the niunber of idants to one half always occurs ; and this, as 

 I have recently attempted to show, must have the eiTect of in- 

 creasing the number of possible combinations of idants very 

 considerably.! 



The facts as they concern the Metazoa may be briefly sum- 

 marised as follows. In all those species which have been in- 

 vestigated for this purpose, the germ-cells are formed by the 

 mother-cell undergoing two consecutive divisions, each of which 

 results in a halving of the number of idants, one half passing into 

 the one daughter-cell, and the other half into the other. In the 

 second division this would lead to the presence of only a quarter 

 of the original number of idants, if the number in the mother-cell 

 were not doubled by each idant becoming split into two before 

 the first division takes place. Thus there is first a doubling, and 

 then a halving, of the number of idants. It is a matter of 

 secondary consideration in the question of heredity that in the 

 formation of the female germ-cell or ovum three of the cells pro- 

 duced by the division of the mother-cell give rise to the evanes- 

 cent '■ polar-bodies,' one cell alone becoming an ovum capable 

 of development, while all four of the male germ-cells become 

 functional. The chief point which now concerns us is the proc- 

 cess of doubling, and the two subsequent halvings of the number 

 of idants : this is known to occur in all classes of the Metazoa 

 from the low-est to the highest forms, and, as far as we know^ is 

 only wanting in those eggs which are adapted for partheno- 

 genesis. Even in these cases the doubling also occurs, but it is 

 followed by only a single halving of the number of idants, in 



* Dr. vom Rath informs me that in the crayfish {Astacus fiiiviatilis) the 

 number of idants reaches 108-125. 



t Cf. 'Amphimixis,' Jena, 1891 (Essay xii. in the Enghsh Translation, 

 Vol. ii., p. 105). 



