CH. II.] 



KARYOKINESIS 



17 



Polar circle 



Medullary 



In cells which are the result of the sexual process, the number 

 of chromosomes is always even, an equal number being contributed 

 by each sex. 



The number of chromosomes varies with the species from four 

 to twenty-four; in man the 

 number is sixteen. 



5. The stage of metakinesis. 

 The sister threads separate, 

 one set going towards one 

 pole, and the other to the 

 other pole of the spindle 

 (fig. 21): these form the two 

 daughter nuclei. The chromo- 

 somes are probably pulled into 

 their new position by the con- 

 traction of the spindle fibres 

 attached to them. 



6. Each daughter nucleus 

 goes backwards through the 

 same series of changes; the 



diaster or double star is followed by the dispirem or double skein, 

 until at last two resting nuclei are obtained (fig. 22). 



A new membrane forms around each daughter nucleus, the spindle 

 atrophies, and the attraction sphere becomes less prominent. The 



Attraction 

 sphere 



Pole-body 



Antipodal zone 



FIG. 20. Ovum of the worm Ascaris in process of divi- 

 sion. The attraction spheres are at opposite ends 

 of the ovum ; at the equator of the spindle which 

 unites them, four chromosomes are seen. The proto- 

 plasm of the ovum, except in the equatorial zone of 

 the cell, is arranged in lines radiating from the centre 

 (centrosome) of the attraction sphere. (Waldeyer.) 



. . Fine uniting 



filaments. 



Fio. 21. Metakinesis. A, Early stage. B, Later stage, c, Latest stage formation of diaster. A and 

 B show how the sister threads disentangle themselves from one another. (Waldeyer.) 



division of the protoplasm into two parts around the nuclei begins 

 in the diaster stage, and is complete in the stage represented in 

 fig. 22. 



The karyokinetic process has been watched in all its stages by 

 more than one observer. The time occupied varies from half an hour 

 to three hours ; the details, however, must be studied in hardened 

 and appropriately stained specimens. They are most readily seen 

 in cells with large nuclei, such as occur in the epidermis of 



B 



