STRUCTURE AND PHYSIOLOGY OF ANIMALS 



17 



these bodies is present at the outset it subsequently becomes 

 divided into two. These are the centrosomes — minute masses of a 

 specially modified protoplasmic substance, capable of being 

 rendered conspicuous by certain staining agents, ' surrounded 

 by a light zone. The centrosomes, at first close together, 

 gradually separate from one another, a spindle-shaped bundle 

 of very fine fibres of achromatic * 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 ; dir. chromatin ; rpl. cell-plate ; mi', nucleoli ; nu. m. nuclear membrane ; *. atrosphere ; 

 *p. spiudle. (From Parker's BMogy, after FIcmming, Habl, &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 



1 The term achromatin is usually applied to all the matter of the nucleus 

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

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



*.. 



