364 INTRODUCTION TO CYTOLOGY 



to lie. Rarely all of the univalents may pass to one pole (megasporocytes 

 of Canince roses). In mitosis II the chromosomes which disjoined as 

 bivalents in / split and separate, as in normal meiosis. The former 

 univalents may be distributed at random as wholes, or they may divide 

 and separate, sometimes even after having divided in /.^ Very often 

 one or more of the univalents may lag behind and fail to be included in 

 either daughter nucleus. They may then degenerate, but often they 

 form small nuclei about which small supernumerary spores ("micro- 

 cytes") are differentiated (Figs. 206, 207). Such cells are functionless. 

 Moreover, many of the spores and gametes which are viable carry very 

 abnormal chromosome complements. Hence plants with such meiotic 

 irregularities are characterized by considerable sterility and the produc- 

 tion of abnormal progeny. 



The union of gametes with altered complements naturally leads to 

 diversity in the following generations. Many combinations do not result 

 in viable forms, but others give rise to types which develop normally until 

 meiosis, when chromosomal aberrations may again appear. In successive 

 hybrid generations meiotic irregularity and genetic instability are 

 observed as long as extra unpaired chromosomes are present. These 

 tend to be eliminated gradually through irregular mitoses, until relatively 

 stable types with one or more numbers of regular chromosome pairs are 

 established. Frequently, in the course of such adjustment through 

 elimination of extra chromosomes, some members of the complement 

 become lost while others are duplicated, so that, although the chromo- 

 some number of one of the original parents may be restored, the assort- 

 ment is a new one and the plants are correspondingly altered in character. 

 This again emphasizes the fact that it is the qualitative constitution of the 

 complement, rather than the mere number of chromosomes, that is of 

 primary significance in species alteration. 



Analysis of Alloheteroploid Types. — The formation of bivalents 

 rather than multivalents in the euploid plants mentioned above is 

 attributed to dissimilarities in the chromosome sets, that is, some of the 

 sets are homologous enough to undergo synapsis, while others are not. 

 The plants in question are alloheteroploid, at least in part. This con- 

 dition is generally ascribed to hybridization. It is probable, however, 

 that in autopolyploid forms differences sometimes develop in the asso- 

 ciated sets through mutation, translocation, and the like, the differentia- 

 tion finally being great enough to prevent synapsis between the members 

 of certain sets. At the same time, there is evidence which points to the 



^ Two successive splittings are reported for Hieracium (Rosenberg, 1917), 

 Raphanus X Brassica (Karpechenko, 19276), and Rosa (Tackholm, 1922; Hurst, 

 1931). The types of behavior on the part of unpaired elements in / are tabulated by 

 Tackholm. 



