i MITOSIS ii 



longitudinally split into two daughter chromosomes. The metaphase in 

 Strasburger's sense is the rather ill-defined moment when the two daughter 

 chromosomes begin to move apart. Very often, however, the word is 

 used to cover the whole period in which the complete mitotic figure 

 persists, with the chromosomes arranged midway between the two 

 poles of the spindle and the daughter chromosomes not yet completely 

 separated. 



At this stage it can be seen that each daughter chromosome is attached 

 by one or more spindle fibres to one (and only one) centrosome. 



The anaphase is concerned with the final separation of the two groups 

 of daughter chromosomes, each of the latter travelling up the line of the 

 spindle fibres towards one of the poles of the spindle. It is generally 

 agreed that the fibres of the achromatic figure are the visible expression 

 of the forces by which the movements of the chromosomes are effected, 

 but there is considerable difficulty in determining the nature of their 

 action (p. 23). 



Often the separation of the daughter chromosomes takes place 'very 

 regularly, so that by the splitting of the individual chromosomes which 

 compose it the metaphase equatorial plate is divided into two daughter 

 plates, which gradually diverge from one another. In other cases the 

 movements of the chromosomes are not so regular, so that the separating 

 daughter chromosomes travel up to the poles more independently. 



The telophase comprises the metamorphosis of each of the two clumps 

 of daughter chromosomes into a new resting daughter nucleus ; the 

 details of this process are discussed below. 



During telophase, or late anaphase, the cell body becomes constricted 

 between the two new nuclei, the constriction becoming deeper and deeper 

 till finally two separate cells are produced, each containing one of the 

 new daughter nuclei. 



Each daughter nucleus thus contains one of the products of division 

 of each of the chromosomes in the mother nucleus. As regards chromo- 

 some constitution, the daughter nuclei are therefore of like constitution 

 with each other and with the mother nucleus. 



A fact of fundamental importance for cytological theory, and one 

 that has been established by innumerable observations, is that, with 

 certain mostly well-understood exceptions which will be discussed in the 

 later chapters of this book, the number of chromosomes in the nuclei 

 of any given species is constant. Thus to take the species whose nuclei 

 are figured in this chapter, the number of chromosomes in the nuclei of 

 Lepidosiren is 38. It is indifferent in what tissue the nucleus is situated ; 

 whether it is a skin, nerve, muscle, connective tissue or other nucleus, 

 the number of chromosomes which it exhibits at mitosis is 38. In 



