I STRUCTURE AND PHYSIOLOGY OF ANIMALS 19 



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 1 material the nuclear spindle 

 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 

 termed the attraction-sphere or astrosphere (Fig. 5, A, s). Meantime 

 important changes have been in progress in the nucleus. The 

 linin network with its attached chromatin granules or chromioles 

 first becomes arranged in a close tangle (spireme), and then becomes 

 divided up in a very definite and orderly manner into a number of 

 p ar ^s the chromatin segments or chromosomes which frequently 

 have the form of loop-like threads (Fig. 5, B, C, chr), but often 

 assume other forms. The number of chromosomes varies, but is 

 constant throughout the cells of the same species of animal. The 

 nuclear membrane disappears. Each of the chromatin segments 

 splits lengthwise into two parts the daughter-segments of the chro- 

 matin or daughter-chromosomes (Fig. 5, B D), and with these the 

 filaments of the spindle become connected. 



At this point the segments of the chromatin form a single 

 group the equatorial plate 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 daughter-segment 

 of the chromatin now separates from its fellow, so that two groups 

 are formed, each containing a similar number of chromosomes. 

 The two groups then move apart from one another, each approach- 

 ing the corresponding end or pole of the spindle with its 

 centrosome (Fig. 5, E G). How this movement is effected 

 is not definitely known ; it has been supposed that it is due to the 

 contraction of spindle-fibres attached to the centrosomes ; but 

 since there is no appearance of the fibres shortening or thicken- 

 ing, it is unlikely that this can be the true explanation. 



When the groups have approached the extremity of the spindle, 

 the segments of each unite, and eventually the entire chromatin of 

 each of the two groups assumes the arrangement which the 

 chromatin of the original nucleus exhibited before division began. 

 A new nuclear membrane becomes formed around each chromatin 

 group, and the whole assumes the character of a complete nucleus 

 the daughter-nucleus (Fig. 5, //, /). It is of importance to 

 note that, though in this mitotic division of the nucleus of 

 the animal cell the centrosomes are so conspicuous that it 



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. 



c 2 



