348 INTRODUCTION TO CYTOLOGY 



chromosomes of one kind (a "trisome"); through ehmination and trans- 

 location the materials of one member of the trisome, except for the 

 kinetic body of the spindle-attachment region, are then replaced by some 

 of the materials of a non-homologous chromosome (or of more than one of 

 these), so that the materials of the latter chromosome come to be associ- 

 ated with two kinetic bodies instead of one; through disjunction at 

 meiosis the normal homologue of the contributing chromosome is elimi- 

 nated, and gametes with two chromosomes in its place are formed; union 

 of two such gametes gives an individual having one more chromosome 

 pair than the original type but with the same assemblage of chromosomal 

 materials. The basic number may also be changed in the opposite direc- 

 tion if translocations and eliminations leave a kinetic body with little or 

 no chromosomal matter, and this body is then segregated out. 



Some of the irregularities described in the foregoing pages, notably 

 asynapsis, lagging, and zygotic doubling, have been observed repeatedly 

 in interspecific hybrids. The hasty conclusion that all such aberrations 

 indicate hybridity has been proved erroneous by what has been learned 

 about the effects of cultural conditions; but it remains true that hetero- 

 ploidy has one of its chief causes in hybridization,^^ as will be made clear 

 in a later chapter. In the study of such phenomena and their significance 

 it therefore becomes necessary to distinguish two kinds of heteroploidy: 

 autoheteroploidy and alloheteroploidy. 



Autoheteroploidy and Alloheteroploidy. — In autoheteroploid types 

 the chromosomes of only one kind of set are involved. For example, a 

 plant with three sets which are alike (within the usual specific limits) 

 is an autotriploid, whereas a plant with two sets from one species and one 

 set from another is an allotriploid. Similarly, a plant with a single one of 

 its chromosomes in triplicate is an "autohyperploid, " whereas one in 

 which the extra chromosome is one from the set of another species is an 

 "allohyperploid. " In other words, the autoheteroploid types are specif- 

 ically "pure" as regards their chromosome complements, while the allo- 

 heteroploid types are "hybrid." It is obvious that such a distinction 

 cannot always be sharply drawn, since hybridity may exist in so many 

 gradations; moreover, both conditions may be present together, as, for 

 example, in a hexaploid having three sets from one species and three from 

 another. Nevertheless, failure to distinguish the conditions as far as 

 possible has brought much confusion into the literature on this subject. 



When the origin of the heteroploid condition is not directly known, 

 an examination of the morphology of the chromosomes composing the 

 complement often yields decisive evidence as to the kind of heteroploidy 

 represented. For example, in euploid forms of Crepis, whose chromosome 



32 Rosenberg (1917), Ernst (1917, 1918), Winge (1917), I. Holmgren (1919), 

 Tackholm (1920, 1922), Jeffrey et al. (1922), Jeffrey (1925a), Afzelius (1924), Gates 

 (1924a, 1925, 1929a). 



