18 ZOOLOGY SECT. 



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

 important changes have been in progress in the nucleus. The 

 chromatin first becomes arranged in a close tangle (spireme), 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. 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 chromatin 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 daughters-nucleus (Fig. 5, H, I). 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 

 would appear as if they had an important share in controlling 

 the process, yet mitosis takes place during cell-division of the 

 higher plants on the same general lines as in animals though 

 centrosomes have rarely, if ever, been observed in plants higher 

 than the Mosses. 



A furrow which appears on the surface of the cell-protoplasm (Fig. 

 5,H,I), surrounding it in the form of a ring in a plane at right angles 

 to the long axis of the spindle, deepens gradually so as to give rise 

 to a cleft, eventually completely separating the substance of the 

 cell into two halves. Each of these halves encloses one of the 

 daughter-nuclei, and has assumed the character of a complete 

 daughter -cell. During this process there is sometimes distinguish- 

 able along the line corresponding to the division line between the 



