86 MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



there is fertilisation there is also reduction, or, as we shall call it, 

 meiosis. The course of this verification has been beset with many 

 difficulties, which will later be considered in detail. They have 

 been discussed at length by several authors (Wilson, 1925 ; Belar, 

 1928 ; Renter, 1930 ; D., 1931 ; v. Appendix I). 



We may now define meiosis by its superficial phenomena as the 

 occurrence of two divisions of a nucleus accompanied by one division 

 of its chromosomes. It results in the production of four nuclei, each 

 of which has half the number of chromosomes of the mother-nucleus, 

 provided that their distribution has been regular. 



The two divisions of the nucleus are accompanied by the usual 

 mitotic mechanism of spindle-formation. The single division of the 

 chromosomes consists in the usual mitotic longitudinal reproduction 

 by which each chromosome produces two chromatids. At once 

 we see therefore that the difference between meiosis and mitosis 

 consists in the external mechanism, which acts twice, getting out of 

 step with the internal mechanism, which acts once. Precisely how 

 the two systems get out of step we shall see from a detailed study. 



From the principles set out in the first chapter, it will be clear 

 that the occurrence of one form of nuclear division instead of 

 another at a particular point in the development of the organism 

 is an example of differentiation — in time, in the Protista, and in 

 time and place, in the higher organisms. This differentiation being 

 a racial characteristic must be genetically determined, although its 

 origin is too remote for its exact conditions to be reconstructed. 

 Organisms do occur, however, in which the reverse change has taken 

 place, and meiosis has been more or less completely replaced by one 

 or two mitoses. These cases show that the two forms of division 

 are distinguished by a simple physiological difference which is 

 controlled by the genetic properties of the organism. The nature 

 of this difference and of this control will be considered later in 

 relation to the theory of meiosis, of apomixis, and of cell-mechanics. 



2. OUTLINE OF MEIOSIS 



The following sketch shows the succession of events in meiosis, as 

 they occur in the simplest clear examples, e.g., pollen mother-cells 



