34 CELL DIVISION [CH. 



A short general account of the process of cell-division has 

 now been given, and it remains to discuss in rather fuller 

 detail some points connected with the behaviour of the 

 chromosomes. The questions involved in the formation of 

 the chromosomes from the scattered chromatin of the rest- 

 ing nucleus will be considered later ; here it will be conveni- 

 ent to deal with the arrangement of the chromosomes on the 

 spindle, and the manner of their division and separation to 

 form the two daughter-nuclei. 



When the nuclear membrane has disappeared and the 

 chromosomes are fully formed, their shape varies greatly in 

 different species of animals, and in some cases there is con- 

 siderable variety among the chromosomes in any one cell. 

 Where there is such diversity, it is often possible to 

 recognise individual chromosomes, or rather corresponding 

 chromosomes, in each mitotic figure, and to see that the 

 same series reappears at every cell-division ; and in a hybrid 

 between two distinct species the chromosomes derived from 

 each parent species may be recognised by their character- 

 istic form. In general the chromosomes have the form of 

 rods of nearly uniform thickness and with rounded ends, 

 but they vary in length from rods so short that they are 

 practically spheres, to elongated bodies whose length is 

 twenty or more times their width. It has been maintained 1 

 that the width of all the chromosomes of any one species, 

 and even any group, of animals, is constant, and although 

 this is almost certainly not true of different kinds of cells, 

 it is generally found that all the chromosomes in anyone 

 mitotic figure have approximately the same width, but may 

 differ in length and in the shapes into which they are bent. 

 Short chromosomes are commonly straight, but the larger 

 ones are usually bent in the form of a V, of which the two 

 1 E.g. by MEEK (191 2). 



