246 THE GERM-PLASM 



In Ascaris megalocepJiala 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 number of idants to one half always occurs ; and this, as 

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

 creasing the number of possible combinations of idants very 

 considerably.! 



The facts as they concern the Aletazoa 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 lyidergoing 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 

 ^vere 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 o\um 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 lowest 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 Jluviatilis) the 

 number of idants reaches 108-125. 



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

 Vol. ii., p. 105). 



