680 SCIENCE PROGRESS 



division of the ovum into two daughter cells and the divisions 

 in the subsequent generations of cells thus produced is a com- 

 plicated process. A cell, as has already been pointed out, is 

 a small mass of protoplasm but it is not a simple homogeneous 

 mass of this substance. Every cell contains within it an area, 

 generally surrounded by a membrane, which is denser and 

 differs in other respects from the rest of the cell. This is the 

 nucleus. The remainder of the protoplasm forming the cell is 

 the cytoplasm. When a cell is about to divide, radiations 

 appear round two centres in the cytoplasm and as the time 

 of actual division approaches these two centres separate more 

 and more widely, the radiations forming a spindle of fibres 

 between them. The whole figure presents an appearance similar 

 to that exhibited by iron filings when spread upon a piece of 

 paper laid over the poles of a magnet, except of course that the 

 figure in the cell is three-dimensional. 



While this has been taking place, certain bodies, chromo- 

 somes, have formed within the nucleus. These bodies contain 

 a substance present in the nucleus in a scattered condition at 

 other times which collects, during the process of division, to 

 form the chromosomes ; this substance is called chromatin, on 

 account of the readiness with which it takes up the dyes used 

 in making microscopic preparations. During all the divisions 

 which take place in the building up and keeping in repair of 

 the bodies of animals and plants, the chromosomes are similar 

 in appearance, their form being that of rods, U's or Vs. They 

 are constant in number in the cells of individual species of 

 animals and plants. Thus in the cells of man thirty-two chromo- 

 somes appear upon division, in the newt twenty-four, in Ascaris 

 megalocepha launivalens (a parasitic worm in the horse) two ; and 

 so on. When the chromosomes are fully formed, the nuclear 

 membrane disappears and each of the chromosomes becomes 

 attached to one of the fibres of the spindle which has been 

 described already and lie flat upon the equatorial plane ; the 

 whole body of the cell acquires an hour-glass shape and the 

 chromosomes split longitudinally, a half of each travelling 

 towards opposite poles of the spindle. As these half chromo- 

 somes collect around the respective poles, the body of the cell 

 divides into two at the constricted point in the middle, producing 

 two daughter cells, each of which contains an exact repre- 

 sentative half of each of the chromosomes present in the parent 



