254 INTRODUCTION TO CYTOLOGY 



(for other elements). Disjunction in I is called "prereduction"; that in 

 //, "postreduction." 



It should be noted that the several tetrads in the first meiotic meta- 

 phase are arranged entirely at random with respect to the poles toward 

 which the paternal and maternal members are directed (see Fig. 149). 

 All of the paternal elements may face one way and all of the maternal ones 

 the other, or any other possible arrangement may obtain ; apparently this 

 is wholly a matter of chance.^ Hence the distribution of chromosomes 

 in meiosis is a random matter so far as their parental derivation is 

 concerned. 



At the close of meiosis each of the four resulting nuclei has a mono- 

 ploid chromosome group (a set) made up of one chromatid of each of the 

 tetrads. In other words, the set comprises a longitudinal half of one 

 member of each of the pairs of homologous chromosomes in the original 

 diploid complement. Thus meiosis involves two nuclear divisions but 

 only one chromosomal division. Representing the three pairs of chromo- 

 somes in Fig. 149 by the letters Aa Bh Cc, it is found that two of the four 

 nuclei have A and the other two a; two have B and two b; two have C and 

 two c. It should be readily understood that the random assortment 

 of the three pairs of parental elements makes it possible for a nucleus 

 of the quartet to have any one of eight possible combinations: ABC, A Be, 

 AbC, Abe, aBC, aBc, abC, abc.^ 



A further point should be noted. Two nuclei having A and a, 

 respectively, may be considered to differ qualitatively according to the 

 degree of difference between these chromosomes. If, in the meiocyte 

 nucleus, the members of this pair are unlike (Aa), two of the quartet 

 nuclei will differ qualitatively from the other two; whereas, if the members 

 are alike (A A or aa) in the meiocyte, all of the quartet nuclei will be alike 

 so far as this pair is concerned. The same holds for all of the pairs. 

 Were the two members exactly alike in every pair, meiosis could occur 

 without producing any qualitative differences whatsoever among the 

 nuclei of the quartet. The essential point to be borne in mind here is that 



^ The same would be true of dyads in the second mitosis in cases of postreduction. 

 Random distribution was first demonstrated with pairs composed of dissimilar mem- 

 bers in the orthopteran genera Brachystola and Trimerotropis (Carothers, 1913 1917). 



^ These statements are made on the assumption that the chromosomes remain 

 individually intact throughout meiosis. When homologous chromosomes exchange' 

 portions with one another, as will be shown later, the statements will still hold for a 

 given small region of a chromosome. 



When all of the chromosomes disjoin in / and divide equationally in //, as shown 

 in the diagram, there are chromosome sets of two types in the nuclei of the quartet, 

 each type being represented twice. When / is equational for portions of tetrads and 

 the dyads are not oriented similarly in the two spindles at //, it is possible to have 

 sets of four types in the quartet. Since the organism usually produces many quartets, 

 there is abundant opportunity for the production of all possible types. 



