4i6 



BREAKDOWN OF GENETIC SYSTEMS 



form a connection between the daughter nuclei, which will fuse to 

 give a restitution nucleus either at first or second anaphase or both. 

 In this last case only a single nucleus is produced with double the 

 somatic number of chromosomes. 



In all these circumstances we see that we are dealing with the 

 varying relationships of the time of division of bivalents and 

 univalents to one another and to the development of the spindle. 

 We also see that while the position of the chromosomes depends 

 in the first instance on the spindle, the changes of shape in the 

 spindle depend on the separation of the paired chromosomes in 

 bivalents and of the daughter chromatids in dividing univalents. 

 The behaviour of dividing cells in which the normal relationships 

 of the spindle to the chromosomes has been upset by failure of 

 pairing is nearly always variable. No doubt this is due to a sus- 

 ceptibility to environmental changes from which the normal process 

 is protected by a wide margin of safety. 



(vi) Non-reduction. The results of a regular failure of the first 

 or second division is the formation of unreduced or approximately 

 unreduced gametes which function and yield polyploid progeny in 

 the case of diploids, diploid progeny in the case of haploids. Some- 

 times failure of both divisions may yield gametes with double the 

 parental number, so that diploids may yield hexaploid and octoploid 

 progeny {e.g., in Raphanus-Brassica and Chrysanthemum) and 

 haploids yield triploid progeny. 



The results of the functioning of polyploid gametes have already 

 been shown in relation to the origin of polyploids. Their effect on 

 the reproduction of haploids is shown in Table 64 to be analogous. 



Table 63 

 Numbers of Cells in '* Tetrads " of Haploid Plants 



Datura (1867) w = 12 

 Matthiola (406) w = 9 

 Crepis (282) w = 3 



Hexads. 



205 



