188 



KARYOKINESIS. 



The filaments have been described in some instances as splitting transversely instead 

 of longitudinally (Carnoy), but it is uncertain if this is really the case. In the division 

 of the ovum of Ascaris megalocephala (fig. 219), v. Beneden describes the achromatic 



Fig. 218. STAGES IN THE DIVISION OF THE OVUM OR EGG-CELL OF A WORM. (Strasburger. ) 



a, ovum before division ; 6, nucleus occupied by a spindle-shaped system of fibres, with chromatin 

 nodules (chromosomes) at the equator of the spindle ; c, separation of chromosomes into two parts which 

 gradually travel towards the poles of the spindle and there become transformed into new (daughter) 

 nuclei, whilst the protoplasm at the same time also separates into two parts (d, c, f) ; in d thickenings of 

 achromatic fibres in middle of spindle, indicating plane of division of cell ; g, repetition of the division- 

 process, formation of spindles in daughter-nuclei ; h, result of the division of these. 



spindle as being preceded by the differentiation within the protoplasm of two 

 "attraction-spheres" which surround the pole-corpuscles, but are separated from 

 them by a clear area. When the spindle is developed its fibrils extend from the centre 



JJ.C, 



Fig. 219. OVUM OF ASCARIS MEGALOCEPHALA IN PROCESS OF DIVISION. 



A, ovum before division, with the chromosomes of the female pronucleus in the skein condition. 

 Adjacent to the pronucleus are two attraction- spheres, united by an achromatic spindle. 



B, the attraction-spheres are now at opposite ends of the ovum : at the equator of the spindle 

 which unites them, four chromosomes are seen. The protoplasm of the ovum, except in the equatorial 

 zone of the cell, is arranged in lines radiating from the centre (central particle) of the attraction spheres. 



C, stage of metakinesis. The chromosomes are seen passing along the achromatic spindle towards 

 its poles ; the equatorial zone is broader, and the attraction lines extend a less distance in the proto- 

 plasm ; the central particle of the attraction sphere is divided into two. 



D, the daughter-cells are now formed, and the attraction-sphere in each is already divided prepa- 

 ratory to the division of the daughter-nuclei. 



e.c., equatorial circle ; p.c., polar cone ; c.c., pole-corpuscle. 



of these attraction-spheres, which thus occupy its poles, towards its equator, and are 

 on the other side prolonged towards the periphery of the protoplasm as a conical 



