CELL-DIVISION. 9 



The notable exception to the constancy of the numerical quota of 

 the chromosomes presented by the germ-cells should be mentioned. Since 

 the chromosomes of the fertilized ovum are derived from the chromatin 

 contributed equally by the paternal and maternal germ-cells the spermato- 

 zoon and the ovum it is evident that unless the number of chromosomes 

 from each parent be only one-half the usual number for the species, the 

 segmentation nucleus and the succeeding cells would contain twice the 

 normal quota of chromosomes. In order to prevent such redundancy, 

 during the development of the spermatic cells, on the one hand, and the 

 maturation of the ovum on the other, reduction of the chromosomes to 

 one-half the usual number actually takes place. The details by which this 

 reduction is accomplished vary in different classes of animals ; but, whatever 

 be the method, the result is to reduce the number of chromatin-masses 

 one-half. This reduction occurs before the first of the divisions, pro- 

 ducing the capable germ-cells. The full quota for the species is restored 

 to the segmentation nucleus and its descendants by the subsequent ad- 

 dition of the reduced contingents of the two germ- cells when fertilization 

 occurs. 



Meanwhile the centrosomes have continued to migrate towards the 

 opposite poles of the dividing cell, where each forms the centre of the astral 

 radiation that marks either pole of the amphiaster. The purpose of the 

 achromatic figure is to guide the longitudinally cleft chromosomes towards 

 the new nuclei during the succeeding changes. 



The Anaphases accomplish the migration of the chromosomes, each 

 pair of sister segments contributing one unit to each of the two groups of 

 chromosomes that are passing towards the poles of the achromatic spindle. 

 In this manner each new nucleus receives not only one-half of the chromatin 

 of the mother nucleus, but also the same number of chromosomes that orig- 

 inally existed within the mother cell, the numerical constancy of the particu- 

 lar species being thus maintained. 



In the beginning of their passage towards the poles of the. achromatic 

 figure, the migrating chromatic segments, attracted along delicate threads, 

 for a time form a compact group about the equator of the spindle. As the 

 receding segments pass towards their respective poles, the opposed ends of 

 the separating chromosomes are united by intervening achromatic threads, 

 the connecting fibres. Sometimes the latter exhibit a linear series of thick- 

 enings, known as the cell-plate or mid-body. The migration of the chromo- 

 somes establishes the essential features of the division of the nucleus, since 

 the subsequent changes are only repetitions, in reverse order, of the details 

 of the prophases. 



The Telophases, in addition to the final stages in the rearrangement 

 of the chromatic segments of the new nuclei, including the appearance of the 

 daughter skeins, of the new nuclear membranes and of the nucleoli, inaugu- 

 rate the participation of the cytoplasm in the formation of the new cells. 

 During these final stages of mitosis, the cell-body becomes constricted and 

 then divides into two, the plane of division coinciding with the equator of 

 the nuclear spindle. Each of the resulting masses of cytoplasm invests a 

 new nucleus and receives one-half of the achromatic figure, consisting of a 

 half-spindle and one of the asters with a centrosome. The new cell, now 

 possessing all the constituents of the parent element, usually acquires the 

 morphological characteristics of its ancestor and passes into a condition of 

 comparative rest, until called upon, in its turn, to undergo division and ente* 

 upon the complicated cycle of mitosis. 



