m] MITOSIS 33 



two, and these begin to separate and the beginnings of the 

 achromatic spindle and astral rays appear. The next phase 

 is the movement of the centrosomes to opposite poles of the 

 nucleus, the formation of definite chromosomes, and the 

 disappearance of the nuclear membrane with the concurrent 

 completion of the spindle and asters. These two stages are 

 spoken of as the propbase of mitosis, the first being the 

 early, and the second the late prophase. The chromosomes 

 then take up their definite position in a circle in a plane 

 transverse to the line joining the centrosomes; this is the 

 equatorial plate stage. The chromosomes divide, so that 

 there is now a circle of double chromosomes in the equa- 

 torial plate, constituting the metaphase, which seems to last 

 for a short time before the halves of the chromosomes begin 

 to diverge towards the poles of the spindle. The chromo- 

 somes then begin to travel along the spindle towards the 

 centrosomes, one longitudinal half of each chromosome 

 going to each pole; this stage is known as the anaphase. 

 Finally, the chromosomes become crowded together at the 

 poles and coalesce to form the daughter-nuclei, and the 

 cytoplasm of the cell begins to divide across the middle of 

 the spindle; this last stage of division is the telophase. 

 Since the same process takes place in every, division of 

 every cell from the fertilised egg onwards (with the excep- 

 tion of the developing germ-cells and certain abnormal 

 divisions, which will be mentioned later) and since in each 

 division the chromosomes are accurately halved by a longi- 

 tudinal split, it follows that every cell of the body contains 

 the same number of chromosomes. And since, as will be 

 seen later, there is reason to believe that the longitudinal 

 splitting divides each chromosome into similar halves, it 

 follows also that each cell contains a complement of similar 

 chromosomes. 



D.C. * 



