300 THE EVOLUTION THEORY 



knowledge of the phenomena of the maturation of the ovum, that 

 a reduction of the chromosomes by half must take place, and had 

 postulated a similar ' reducing division ' for the sperm-cell, and 

 further, for plants as well as animals — indeed, for all sexually repro- 

 ducing forms of life. The two divisions in the sperm-cell corre- 

 sponding to the polar divisions of the ovum with their reduction 

 of chromosomes were demonstrated by Oscar Hertwig in the case 

 of the thread-worm of the horse (Ascaris megalocephala) — a form 

 which has proved so very important in relation to the whole theory 

 of fertilization. It is true that in this case the course of the pheno- 

 mena of reduction is less convincing than in some other forms which 

 have been investigated more recently, as, for instance, the mole-cricket 

 and the bugs. In these instances, at any rate, a ' reducing division ' 

 in spermatogenesis, quite corresponding to that of the egg-cell, has 

 been demonstrated, and this demonstration is of particular value 

 owing to the fact that the development of the sperm-cell, as we shall 

 presently see, throws an entirely new light on that of the ovum, and 

 especially on the phyletic significance of the polar bodies. 



We began our consideration of the processes of reduction with 

 the full-grown egg-cell, but now let us go back to the earliest 

 rudiments of the ovary of the embryo, and we find that it consists 

 of a single primitive egg-cell, from which, by division, all the other 

 egg-cells arise. In the same way the first rudiment in the testis or 

 spermary is formed by a primitive sperm- cell, which does not differ 

 visibly from the primitive egg-cell. Both now multiply by division 

 for a considerable time, and in the ovary this is followed by a period 

 of growth, during which multiplication ceases, and each cell increases 

 considerably in size and lays in a store of yolk. Each cell thus 

 ultimately reaches the condition with which we started previously, 

 that of the full-grown mother-egg-cell. 



Although the primitive sperm-cells do not exhibit such pronounced 

 growth as the ova, they have likewise their period of growth, during 

 which multiplication hy division ceases, and the cells increase only in 

 size (Fig. jj, A). When they have attained their maximum of growth 

 the number of chromosomes is seen to have been doubled by longi- 

 tudinal splitting (as in the diagram, Fig. 77, B, from four to eight). 

 From this mother -sperm-cell there now arise by two divisions in 

 rapid succession (G-F) four sperm-cells, and the same reduction of the 

 number of chromosomes to half is effected as in the polar divisions of 

 the egg-cell. In the first division, four chromosomes go to each 

 daughter-cell (D), in the second, two (F). The only essential difference 

 between the corresponding processes in the egg-cell and the sperm- 



