FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 315 



of them, the fertilizing nucleus, which immediately moves towards 

 the ovum-nucleus and apposes itself closely to it. Then follows the 

 fusion or conjugation of the two nuclei, which are alike in size and 

 appearance, just as in the fertilization of the animal ovum «\ £l and 

 ?*). Whether in this case, too, the sperm-nucleus brings with it 

 a central corpuscle, or whether, as Guignard believed he observed, the 

 ovum retains its central corpuscle (C, csph), or finally, whether both 

 modes occur, is not yet known with certainty. The fact that, as 

 a rule, seeds capable of reproduction only form in an ovule when the 

 stigma has been previously dusted with pollen, leads us to suppose 

 that, in this case, as among animals, the ovum lacks something that 

 is necessary to induce embryonic development, only retaining this 

 power in very exceptional cases, namely, when adapted for partheno- 

 genesis. And this something may very well be the dividing apparatus 

 of the cell, the centrosome with the centrosphere. But whether this 

 supposition prove correct or not, a nuclear spindle always forms 

 simultaneously with the fusion of the two sex-nuclei into a se^menta- 

 tion nucleus, and this spindle is the starting-point of the young plant. 

 thus exactly corresponding to the first segmentation of the animal 

 ovum. It agrees with it also in the important respect that it again 



contains the full number of chromosomes — twenty-four in the lily 



while the two nuclei, male and female, only exhibit half the number 

 each, that is, twelve. 



Thus a reduction in the number of chromosomes to half takes 

 place in plants also, but it is not yet known with certainty whether 

 this is brought about in the same way as among animals, namely, by 

 reducing divisions. Without entering more fully into this still 

 unsolved and very complex problem, I should like to state that 

 I consider this very probable; indeed, I agree with the view of 

 V. Hacker 1 , that the reducing divisions of plants are only more 

 difficult to recognize as such, and, furthermore, are often disguised by 

 the fact that they often occur alongside of, or between divisions which 

 are not reducing. If it were possible to reduce the number of chrom< >- 

 somes in a cell to half without the aid of cell-division, if, for instance, 

 only half were to integrate again from the chromatin-network, this 

 must have been quite as possible in the case of animal cells, and then. 

 moreover, the single chromosome would not have had the significance 

 of an individuality, and no special form of nuclear division would 

 have been introduced to reduce their number. That it has been 

 introduced seems to me to prove that it was necessary, and since it 



1 See V. Hacker, Praxis und Theorie (lerZellen- und Befruchtungskhre, Jena, 1899. pp. 144-5. 



