88 THE MEIOTIC PHASE [CH. 



experimental support it is not necessary to consider them 

 here. The most probable explanation is that the second 

 maturation division represents the completion of a process 

 which began at a much earlier stage, and that the heterotype 

 mitosis is a peculiar form of division intercalated between 

 the prophase stages and the completion of an ordinary 

 nuclear division. In an ordinary somatic division the 

 chromosomes emerge from the nuclear reticulum and con- 

 tract to form rods which are twice as thick as the chromo- 

 somes in anaphase, so that if the width of the chromosomes 

 in anaphase is taken as the standard or unit, those in the 

 prophase may be regarded as having twice the normal 

 thickness. They then split longitudinally, and the two 

 halves, now of the standard width, separate to the two 

 poles of the spindle. In the meiotic phase the same process 

 takes place, with the addition that the chromosomes of 

 double width are now associated in pairs, so that each 

 bivalent consists, actually or potentially, of two double rods 

 lying side by side. The heterotype division separates the 

 members of the pair, but since each member is of double 

 width, and has often already a visible longitudinal split, 

 the second (homotype) division is needed to complete the 

 process and produce chromosomes of standard or unit 

 thickness. This is illustrated diagrammatically in Text- 

 fig. 12. That this is the true explanation of the double 

 division in the maturation of the gametes is strongly indi- 

 cated by the phenomena found in most parthenogenetic 

 eggs, and in the spermatogenesis of the Hymenoptera, 

 which will be considered in a later chapter. In these cases, 

 for reasons which will be explained, the reduction process 

 is omitted; there is no pairing of chromosomes and the 

 maturation consists of a single equational or homotype 

 mitosis. 



