302 EMBRYOLOGY 



Then the sperm mother cell is split up by two divisions, which follow 

 upon one another without a resting stage being interposed, first 

 into two daughter cells (Fig. iv), and immediately by a second di- 

 vision which causes the actual reduction into four granddaughter 

 cells of equal size. By these processes, each of the two tetrad groups 

 (Fig. 2, ch, and Fig. 3) divide into two pairs of chromosomes, which 

 are shared by the two daughter cells (Fig. 4). Then each pair of 

 chromosomes (Fig. 5, ch) falls again into its individual elements, 

 which are taken up by the granddaughter cells (Fig. 7). These, 

 therefore, contain, as does the mature egg and the polar cells, only 

 a single chromosome from each tetrad group, altogether only two 

 (Figs. 7, 8). Their nuclei are, therefore, reduced to half-nuclei. 



Many will have asked, what aim this noteworthy reduction of 

 the chromatin, which constitutes the important process of egg- and 

 sperm-ripening, may have. The explanation is easily seen if we 

 consider, in connection with the chromatin reduction, the succeed- 

 ing fertilization, and consider that by this a second nucleus is brought 

 into the egg, which combines with the egg-nucleus and thus doubles 

 its chromatin mass. Thus from two half-nuclei a complete nucleus 

 is again formed, from which then arise all the nuclear generations 

 of the new being. Thus ripening and fertilization of the egg stand 

 to one another in a supplemental relation. That fertilization is 

 needed to replace the chromatin reduction may be proved by a con- 

 sideration. 



As the numerical law of the chromosomes has taught us, chro- 

 matin is a substance which shows a tendency to be constant in a 

 given species, not only in relation to its mass, but also in regard to 

 the number of chromosomes in which it splits during karyokinesis. 

 Thus it is a substance which after cell-division increases to the 

 double and is then halved by division, etc. If we now consider that 

 the process of reduction did not occur, then by fertilization two 

 complete nuclei would be united, and the result would be a doubling 

 of the chromatin, in relation to the normal. By every new sexual 

 generation the same process would be repeated, and thus in the 

 course of generations a summation of nuclear substance would be 

 brought about, which in a short time would lead to such a lack of 

 relation between it and the protoplasm, that the contents of a cell 

 would no longer have room for it. 



Led by similar considerations we may say: By the reduction 

 which precedes fertilization the summation of the nuclear mass and 

 the number of chromosomes to the double and multiple which is 

 normal for the species under consideration, is hindered in the sim- 

 plest way possible. 



Thus, the phenomenon of reduction is a general biologic law of 

 the greatest value. What has been observed in one species of animal 



