1) The progress in the chiasma-type theory and the studies of tetrad analyses in lower plants have made clear that each locus in a given chromosome has its specific frequency of the equational separation at the first division of meiosis.<br>But some additional statistical theories whould be necessary to make the state of the chromosome reduction clearer.<br>In this paper following two problems were dealt with:<br>(a) To obtain the frequency distribution of the length of pre- and post- reductional segments in the chromosome tetrads.<br>(b) To calculate the mean length of such segments for some genetically well studied chromosomes.<br>2) For telomitic chromosomes with genetical length of 100l units and without chromatid interference, the frequency distribution of the length (ξ) of post-reductional segments is given by the following formula, if multiple cross-overs are negligible in frequency.<br>Φ(ξ)=φ'(ξ)+2φ'(l-ξ)-(l-ξ)φ”.ξ)-1, <br>where φ(ξ) is the recombination probability between two genes which are 100ξ units apart.<br>For X-chromosome of Drosophila melanogaster (70 units in length), the above formula becomes<br>Φ(ξ)= cos2ξ+2cos(1.4-2ξ)+(1.4-2ξ)sin2ξ-1, <br>the graph of which is shown in Fig. 2.<br>3) Taking the chromosome as the abscissa and assuming that f(x) represents the probability of the post-reduction of a point x, then the mean length of post-reductional parts of the chromosome or chromosomal segment ab is<br>L=∫^b_af(x)dx, <br>which equals the area bounded by the curve Y=f(x), the axis of x and the ordina_??_es x=a, and x=b (cf. Fig. 6).<br>4) The proportions (in percent) of the length of pre- and post-reductional segments calculated by the above method are listed for some genetically well studied chromosomes as follows:<br>l-L/l L/l<br>Neurospora crassa, sex-chromosome 66.5% 33.5%<br>Sphaerocarpus Donnellii, <br>squamifera-chromosome 52.2% 47.8%<br>Drosophila melanogaster, <br>X-chromosome {46% 54% 48%_??_ 52%_??_<br>II-chromosome 48% 52%<br>Drosophila virilis, <br>X-chromosome {40.7% 59.3% 40.0%_??_ 60.0%_??_<br>In this table l-L/l and L/l represent respectively the proportion of pre- and post-reductional segments and the values with the asterisks were calculated on the cytological maps, while other values were all calculated on the genetical maps.