16 ZOOLOGY SECT. 



solid mass of chromatin, or of a substance achwm a tin differing 

 somewhat from chromatin in its properties and less strongly 

 affected by staining agents. Allied to the achromatin of which 

 such nucleolar clumps may be composed is a constituent of the 

 nucleus to which the name of linin is applied. This assumes the 

 form of a network of delicate threads linin filaments which 

 usually have associated with them, embedded in their substance 

 or adhering to their surfaces, rows of chromatin granules, the 

 interstices being filled with a granular material the nuclear sp. 

 When the nucleus divides during the process of division of the 

 cell, its contents, more particularly the chromatin, in many cases 

 go through a remarkable series of changes, to which the term 

 I'd i'ij oldncsis or mitosis is applied. 



At the time when this mitotic division is about to be initiated 

 either one or two minute bodies (Fig. 3, A, c) are to be distinguished 

 situated close together in the cytoplasm in the immediate neigh- 

 bourhood of the nucleus. These are the centrosomes minute 

 masses of a protoplasmic substance which seems to resemble the 

 matter of the nucleolus. The centrosomes, at first close together, 

 gradually separate from one another, a spindle-shaped bundle of 

 very fine fibres of achromatic l material the nuclear spindle ex- 

 tending between them (Fig. 3,C). At the same time each centrosome 

 becomes the centre of a system of fine achromatin fibres (ap- 

 parently made up, like the fibres of the spindle, of rows of granules) 

 which are arranged round it in a radiating manner forming a 

 structure termed the attraction-sphere or astrosphere (Fig. 3, A, s). 

 Meantime important changes have been in progress in the nucleus. 

 The chromatin first becomes arranged in a close tangle, and then 

 becomes divided up into a number of parts the chromatin segments 

 or chromosomes which frequently have the form of loop-like threads 

 (Fig. 3, C, chr\ but often assume other forms. The nuclear mem- 

 brane disappears. Each of the chromatin segments splits length - 

 wise into two parts the daughter-segments of the chromatin or 

 daughter-chromosomes (Fig. 3, D, E), and with these the filaments of 

 the spindle become connected. 



At this point the segments of the chromatin form a single group 

 the equatorial ptfc extending across the 'axis of the spindle. 

 The latter has shifted its position, so that its fibres now run 

 across the original site of the nucleus, each of them having 

 become interrupted and divided into two halves, each of which 

 extends inwards from the corresponding centrosome, and has 

 become connected with one of the daughter-chromosomes. The 

 spindle-fibres now contract, and, apparently as a result of this con- 

 traction, half the daughter-chromosomes become drawn towards 



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

 that has not the special characteristics of chromatin ; but it applies to cytojt?rixnn'<- 

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



