STRUCTURE AND PHYSIOLOGY OF ANIMALS 17 



these bodies is present at the outset it subsequently becomes 

 aivided into two. These are the centrosomes—mimxie masses of a 

 specially modified protoplasmic substance, capable of being 

 rendered conspicuous by certain staining agents, ■ surrounded 

 A^ ^^^ ^°°^' "^^^ centrosomes, at first close together, 

 gradually separate from one another, a spindle-shaped bundle 

 01 very fane fibres of achromatic i material— the nuclear spindle 



Fig. 5. — Diagrams illustrating karyokinesis. A, the resting cell ; B, C, D, successive phases in 

 the formation and arrangement of the chromatin loops and of the nuclear spindle ; E, F ,G, 

 separation of the two sets of daughter-chromosomes and their passage towards the poles of 

 the spindle ; H, I, division of the cell-body and formation of the two new nuclei ; c. centro- 

 some ; chr. chromatin ; c-pl. cell-plate ; nu'. nucleoli ; na. m. nuclear membrane ; s. atrosphere ; 

 sp, spindle. (From Parker's Biology, after Flemming, Rabl, &c.) 



— extending between them (Fig. 5, C). At the same time, 

 or at an earlier stage, each centrosome has become the centre 

 of a system of fine achromatin fibres (apparently made up, 

 like the fibres of the spindle, of rows of granules) which are 

 arranged round it in a radiating manner, forming a structure 



^ The term achromatin is usually applied to all the matter of the ntidevs 

 that has not the special characteristics of chromatin ; but it applies to cytoplasmic 

 structures — i.e. structures belonging to the body of the cell — as well. 



VaL. I C 



