PROTOPLASMIC FUNCTION 45 



perhaps only a thread connects the two new daughter-cells. Finally, 

 this is cut quite through and the process is complete, requiring in ameba, 

 where it may in rare instances be observed, about three hours. Some- 

 times the course of the division is interrupted and much disturbed, as 

 it were, by confusion or disagreement among different parts of the 

 cytoplasm, and there may be long delays seemingly at any stage of the 

 process, but especially during its later stages. From this or some other 

 cause, the nucleus often breaks up into many daughter-nuclei, as, for 

 example, in the giant-cells, which may break up then into several new 

 cells containing the nuclei near their peripheries (Arnold). 



MITOSIS. Mitosis, karyokinesis, indirect nuclear or cell-division, or 

 nuclear segmentation, is the complicated process by which the great 

 multitude of cells divide, for amitosis is decidedly the exception in the 

 animal world. The simple purpose of this marvellously complex process 

 is to divide equally the essential parts of the nucleus and of the cyto- 

 plasm of the mother-cell among the two daughter-cells. As will be 

 recalled (see page 23), a typical cell-nucleus has, besides its linin retic- 

 ulum and nuclear sap or enchylema, numerous masses of nuclein irregu- 

 lar in shape (but usually elongated), of chromatin termed chromosomes, 

 and one or more nucleoli. The cytoplasm, besides its foam-like probable 

 reticulum and enchylema, has in it (at least during its periods of repro- 

 ductive activity and perhaps at all times) a small body called the attrac- 

 tion-sphere, at the centre of which is a minute round body known as the 

 ccntrosome. Yatsu has shown this to be an organ independent structu- 

 rally of both nucleus and cytoplasm, although derived from the latter, a 

 third constituent of the cell; and it is thus that we have classed it above. 

 Its exact status is still in doubt. 



The process of mitosis, remarkably constant in the thousands of 

 animal species and cells, may be seen graphically represented in the 

 diagrammatic Fig. 19, taken from Flemming. The attraction-sphere 

 is, in the resting cell not about to divide, either in the nucleus or 

 close to the nucleus in the cytoplasm, and inconspicuous probably out- 

 side and at one end of the latter. When the process of mitosis is about 

 to begin the centrosomes separate within the attraction-sphere and soon 

 divide the latter into two parts, connected by fibrils. Each becomes the 

 center of numerous rays which extend outward in all directions, con- 

 spicuously through the cytoplasm, the whole of each being called an 

 aster. Meanwhile the masses of chromatin of the nucleus have arranged 

 themselves into lines or threads (A) (hence the name mitosis, given by 

 Flemming), or perhaps into one thread, coiled on itself within the 

 nucleus. This soon breaks up into particles called chromosomes. The 

 number of these chromosomes is constant for one animal species 

 twenty-four in the mouse, salamander, and trout; sixteen in the guinea- 

 pig, ox, and man; sometimes they are only four, or even two. O. 

 Hertwig is of the opinion that the nucleolus or nucleoli divide into parts 

 and are distributed about the chromatin masses. By the time that the 

 chromatin thread (skein) is divided into chromosomes, these latter have 



