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 chromatiii 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 ; cpl. cell-plate ; me', nucleoli ; nu. in. 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 



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 cytoplasmic 

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



VUL. I C 



