.■298 THE EVOLUTION THEORY 



that this cHiual division is of the most ftir-ivachino- importance, for it 

 shows VIS tliat the so-called process of fertilization is the miion of 

 etpial ([uantities of hereditary substance of paternal and maternal 

 oriirin. 



'I'hr process of fertilization is now known in all its details in 

 a ^rcat number of animals in the most diverse groups ; it is every- 

 where the same in its essential features ; there is always only one 

 sperm-cell which normally enters into conjugation with the ovum- 

 nucleus, and in every case the sperm-cell, however minute it may be 

 to begin with, forms a nucleus nearly or exactly as large as the 

 nucleus of the ovum, and in all cases it contains the same number of 

 chromosomes as the nucleus of the ovum. Of special interest, how- 

 ever, is the fact that this number is always half the number of the 

 chromosomes exhibited by the somatic cells of the particular animal 

 in question, and that the reduction of the number of chromosomes 

 to half the normal is effected in both male and female germ-cells by 

 the last divisions of these cells, wdiich take place before they have 

 attained to a state of maturity. In the ovum the reduction occurs in 

 the directive divisions, to wdiich we must therefore turn our attention 

 once more, with special reference to the number of chromosomes. 



We saw that, in the full-grown ovarian egg, the germinal vesicle 

 rises to the surface and there becomes transformed into the first polar 

 spindle. Now this shows, in its equatorial plane, double the numlier 

 of chromosomes normal to the species. This duplication comes about, 

 not directly before the nuclear division, but much earlier in the young 

 mother-egg-cell ; it is only the change in the time of the splitting of 

 the chromosomes that is unusual. The first maturation division takes 

 place nevertheless in accordance with the usual plan of nuclear 

 division ; it is, as I have called it, an ' equation division,' that is, both 

 daughter-nuclei again receive the same number of chromosomes as 

 the young mother-egg-cell had to start with, namely, the normal 

 number of the species. Thus, if the young mother-egg-cell had four 

 chromosomes (Fig. 76, A), this number would double to eight at an 

 early stage (B), but the first maturing division w^ould give each 

 daughter-nucleus four (C and D). In the second maturation division 

 the case is different, for here no splitting and duplicating of the 

 number of chromosomes takes place, but the existing number, hy 

 being distributed between the two daughter-nuclei, is reduced to half 

 in each (E and F). For this reason I have called it a ' reducing 

 division.' In our example, therefore, the ovum, as well as the second 

 polar body, would contain only tw^o chromosomes (Fig. y6, F). 



I cannot enter into the details of the process here, for we are 



