THE VITAL PHENOMENA OF THE CELL 



235 



IV. Division with Reduction. 



During the final development of ova and spermatozoa, certain 

 peculiar processes of division occur, which have for their function 

 the preparation of the sexual cells. The essential characteristic of 

 this is, that in the double division that occurs the second follows 

 the first so quickly, that the nucleus has no time to enter the rest- 

 ing condition. The result is, that the groups of nuclear segments 

 produced by the first division are immediately split up into 

 two daughter-groups without previously undergoing longitudinal 

 cleavage. Hence, at the end of the second division, the mature 

 ego;- and sperm-cells only contain half the number of nuclear 

 segments, and half as much nuclem substance, as are present in 

 the nuclei produced by ordinary cell division in the same animal 

 (Hertwig VI. 34). To this phenomenon the name of "division 

 with reduction" has been given (Weismann VI. 77). Division 

 with reduction is most easily followed in the sperm- and egg-cells 

 of Ascaris megalocephala. 



In the testis tube a certain number of cells are differentiated off 

 to form the sperm-mother-cells. In the large vesicular nucleus 

 (Fig. 125 /.), eight long nuclear threads develop out of the 



IV. 



I. 



II. 



III. 



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On ^ 



o 'T s ^ 

 s o o 00 

 o o o 

 il . u o 



Q 







44 ii 



c 



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Fig. 125. Four nuclei of sperm-mother-cell.s of Ascaris megalocephala bivalens at various- 

 stages of preparation for division. 



chromatin substance. (Ascaris megalocephala bivalens has been 

 selected for description.) These are arranged in two bundles, and 

 are connected with the nuclear membrane by linin threads, which 

 stretch out in every direction. Whilst the nucleolus splits up 

 into separate spherules, two centrosomes, surrounded by a small 

 radiation sphere (Fig. 125 II.), make their appearance near to 

 one another in the protoplasm, close to the outer surface of the 

 nuclear membrane (Fig. 125 II). The segments then become 



