i 9 8 SCIENCE PROGRESS 



this is still a convenient name to use for the material of 

 which the fibrillar network is mainly composed, for at least 

 it serves to remind the student that it is this substance which 

 bestows upon the nucleus its remarkable affinity for the dyes 

 employed to throw it into view. 



An important part which the nucleus plays in the life 

 history of a cell was discovered by those early observers 

 who had the good fortune to be the first to watch the 

 division of a cell into two daughter cells from start to finish ; 

 they saw that the division of the cell was preceded by the 

 division of the nucleus, and for many years the nucleus was 

 regarded as the initiator of the process of multiplication. 

 Much labour was expended in tracing the microscopic patterns 

 (skeins, stars, rosettes, etc.) into which the primary threads 

 or chromosomes are thrown, and it was found that each of 

 these chromosomes divides longitudinally into two sister threads 

 which gradually disentangle themselves from one another 

 and form the basis of the two new nuclei. Exceptions later 

 were found to this rule, and heterotype mitosis is characteristic 

 of a certain stage in the development of the sexual elements, 

 as well as of the cells which make up the growths that are 

 called malignant. 



Within comparatively recent years it has been found more- 

 over that the nucleus is after all but the second in command, 

 and that the primary impulse to cell cleavage is given by 

 the division into two of a tiny particle called the centrosome, 

 which the earlier workers had missed. The two centrosomes 

 so formed retire to opposite poles of the nucleus, and the 

 attractive force they exercise is seen in the radiating lines 

 of protoplasmic threads and granules in their neighbourhood. 

 These remind one very forcibly of the lines which one sees 

 radiating from the poles of a magnet when one places it on 

 a flat surface powdered over with iron filings. Whether the 

 attractive force of the centrosomes is magnetic or something 

 akin to magnetic is but a speculation, but the force is undoubted, 

 for finally the sister threads of the nucleus accumulate around 

 the centrosomes, and last of all the cell protoplasm follows 

 suit and two new independent cells are formed. 



That the chromosomes are of vital importance to the 

 organism is clearly shown by several facts. For instance, 

 they are constant in number not only in the cells of the body, 



