688 HEREDITY AND CELL DIVISION 



MITOSIS 



We have already mentioned that the active or dividing nucleus 

 of a cell of a given species contains a set of chromosomes which 

 are characteristic in number and appearance (Fig. 535). It is 

 obvious that for this constant arrangement to be maintained 

 there must be some special mechanism, and in fact in all normal 

 cell division each chromosome divides longitudinally into two. 

 Further, no chromosome ever arises, so far as is known, except 

 by the division of a pre-existing chromosome. The process of 

 division is called mitosis (formerly also karyokinesis) , and will 

 now be described (Fig. 536). 



9 d 



/A. /.^ 



Fig. 535. — The chromosomes of the body cells of fruit-flies (Drosophila). Here 

 there are only eight chromosomes, and the members of each of the pairs 

 which separate at the reduction division can be recognised by their shapes 

 and sizes. — After Morgan and others. 



2, The chromosomes of the female ; ^, those of the male ; X, x-chromosomes ; Y, a chromosome which 

 forms a pair with the single x-chromosome of the male, but is distinguishable from the latter by 

 having a hooked end. 



The first sign of activity in the nucleus is the division of the 

 centrosome (p. 502), the halves of which travel to opposite 

 poles of the nucleus. Meanwhile the chromosomes appear as 

 relatively long, stainable threads ; close examination shows that 

 each is double, and the half threads are called chromatids. 

 The chromosomes shorten by contracting into a solenoid, and in 

 so doing become thicker ; the nuclear membrane disappears, so 

 that they lie free in the cytoplasm. This is the end of the first 

 stage of mitosis, called prophase. The beginning of the next stage, 

 or metaphase, is marked by the appearance of the spindle, a 

 clear and unstainable region which lies across the cell between the 

 centrosomes. The chromosomes, now fully contracted, come to 

 lie in a flat plate across its equator. Later behaviour shows that 

 each chromosome is attached to the spindle at one point, and it 

 is the position of this which gives the characteristic shape to 



