18 



THE ANIMAL CELL 



[CH. II. 



some. The division of the centrosome of the original cell, and then 

 of the attraction sphere into two, usually precedes the commence- 



A B 



^ r-^ 



Achromatic spindle 



r- Pole of spindle. 



--- Outer granular 

 zone. 



JM.---- Split fibres. 



Inner clear zone. 



Polar corpuscle. 



FIG. 22. Monaster stage of karyokinesis. 

 (Waldeyer.) 



FIG. 21. Later condition of the skein staae in karyokinesis. A. The thicker primary fibres or chromo- 

 somes become less convoluted and the achromatic spindle appears. B. The chromosomes split into 

 two and the achromatic spindle becomes longitudinal. (Waldeyer.) 



ment of changes in the nucleus ; the two attraction spheres become 



prominent in cell division, and the 



connecting achromatic spindle is 



probably also formed from them 



or from the achromatic material 



of the nucleus. 



At this stage the nuclear 



membrane is lost, and thus cell 



protoplasm and nuclear sap 



become continuous; the proto- 

 plasm immediately around the 



nucleus is clear; outside this is 



a granular zone, and here the 



granules are arranged radially 



from the polar corpuscles. The 



star-like arrangement of these granules is much better marked in 



embryonic cells, indeed the 

 lines present very much the 

 appearance of fibrils (see fig. 

 23). 



The V-shaped chromosomes 

 sink to the equator of the 

 spindle, and arrange them- 

 selves so as to project hori- 

 zontally from it. 



In cells which are the re- 

 sult of the sexual process, 

 the number of chromosomes is 

 always even, an equal number 

 being contributed by each sex. 

 The number of chromo- 

 somes varies with the species 



from four to twenty-four ; in man the number is sixteen. 



Attraction 

 sphere 



Pole-body 



Antipodal zone 



Fio. 23. 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.) 



