The causal sequence of meiosis
• . i . • pro6eeds"., and where crossing-over takes place. These prime wriables ..at6 ghe.eontaet-point, the th~e-lhnit of pairing and. the torsion in paired regions. Their action has been inferred from the comparison of pairing in ~dated forms witt{ variabie pairmg frequencies and ohiasma distribution ; from ~he pa.h-ing in hybrids, mutants and po]yploids; and h'om the ~ifieiaI !n}errnption of normal pairing. Suc]a comparisons have to be m~de ~vifla even ~ea te r accuracy and detgii if gee theory of meiotic p~mng.as to be ~ested and consolidated. The time has now arrived for an exac$ definitio~ of the individual aoti.ons and of the interactions of the t~{'egva.riables and of {Heir effect onohiasma formatim.~. k it}~iJ.y of pagfing variM;ion rest~Ith~g from genetic aad :from en~dr0nmental Souzces, i o an interspeeitie eo.mparison wibh varied environmental eoi~Ntigns,i.forms ~ahe most promising approach. ~,r,gt~igcvria., which h.a,s proved such Useful material in the discovery of ~he ele.me~lts o:[ mitosis and . . . . meiosis, is eminent.]y st~ita, ble :for further ana.].gsis} :[or the following rea.soJ.is:
The determination of position in crossing-over
6. SummaryThe positions of chiasmata at metaphase are dependent on (a) their positions at the time of formation, and (b) subsequent movement. These two factors may now be separated analytically.A method is given for determining the mean positions of formation of chiasmata relative to the centromere and to one another, by comparison between the size classes in organisms with a large chromosome size range. The original positions as determined in this way seem to be retained at metaphase. There is thus no need to suppose that interstitial chiasmata have moved between their formation and the attainment of metaphase.Thus terminalization is an all or none process. Either all the chiasmata terminalize or they do not move, except for occasional movement of the most distal chiasma to the end of the bivalent as a result of absorption of the free distal arm.Some properties of chiasma formation in two-armed chromosomes are discussed and certain observations which would be critical for the hypothesis of centromeric determination of the position of chiasmata are described.
Chromosome behaviour in relation to genetics ofAgave
Summary1. The somatic chromosome complements of seven species and the meiotic behaviour, in the pollen mother cell divisions, of five of these are described. A basic chromosome number ofx=30 is inferred.2. The species most widely cultivated for fibre production are shown to be autopolyploids (A. sisalana Perrine (5x),A. fourcroydes Lem. (5x),A. cantala Roxbg. (3x)).3. Breeding experiments indicate that the species are genetically heterozygous; structural hybridity and chromosome differentiation is indicated in the meiotic behaviour.4. A certain degree of relationship between the species in the group Rigidae, presumed from their similarity in general characteristics and limited distribution, is supported by a comparison of their chromosome complements and by limited evidence from breeding experiments. The diploid species will cross with one another and with polyploid species.5. Sterility in some of these species is shown to be in part functional and in part influenced by external conditions. The partial sterility of diploid species requires further investigation.