42 THE EVOLUTION THEORY 



irregularly intermingled, then at the formation of the next nuclear 

 spindle an entirely new arrangement of the halves would result. But 

 in animals, at least, this is certainly not the case ; the processes of 

 reduction are restricted to the maturing divisions. 



Years ago Ischikawa observed that, in the conjugation of 

 Koctiluca, the nuclei of the two animals become closely apposed, but 

 that they do not fuse, although they behave like a single nucleus in 

 the division which follows. In this case paternal and maternal 

 nuclear t<iihstances remain separate (Fig. 83, vol. i. p. 317). The 

 same phenomenon has since been repeatedly observed in many-celled 

 animals, first by Hacker, then by Kuckert in the Copepods, and after- 

 wards b}^ Conklin in the eggs of a Gastropod [Crepidula). But all 

 these observations referred onl}' to the earlier stages of ovum-segmen- 

 tation up to twenty-nine cells, and it could not be affirmed that the 

 distinctiveness of the paternal and maternal chromosomes lasted till 

 farther on into the ontogeny. Professor Hacker now informs me, 

 however, that he has been al>le to trace this separateness in 

 a Copepod (Canthocamptiiti) not only from the beginning of seg- 

 mentation on to the primitive genital cell, but also through the 

 divisions of this up to the mother-egg-cell '. Thus we may now 

 assume that the paternal and maternal hereditary bodies remain 

 distinct, not only for a time, but throughout the whole development, 

 a fact which confirms our assumption of the independence of the 

 nuclear rods, notwithstanding their apparent breaking-up in the 

 nuclear reticulum of the ' resting ' nucleus. This new knowledge 

 throws fresh light in another direction : it proves to us that the 

 remarkable and complicated processes which go on in the nucleus 

 during the maturing divisions ha^e really the significance which I 

 long ago ascribed to them -, that of effecting the maximum diversity 

 of intermingling of the paternal and maternal hereditary elements. 

 For Hacker has shown that dvirino- the second maturation -division 

 the paternal and maternal chromosomes are no longer united each in a 

 special group, but occur scattered about in the nucleus, and subsequently 

 come together again to form two differently combined groups. 



If this were not so, if the maternal and paternal chromosomes 

 remained separate, then the reducing division would cause only one of 

 these groups to reach each of the germ-cells, and thus each mature 

 ovvnn or sperm-cell would contain either only paternal or only 

 maternal hereditary bodies. But this would make a revei'sion to 



^ Since this was written Hackfi- lias publislied his results, ^ee Anatom. Anzeigerxv 

 (1902), p. 440. 



^ See my essay, Amphimixis, Jena, 1891. 



