ME IDS IS 103 



two hoinologucs iiifluciu'C' the saiiu^ grouj) of reactions and characters 

 in tlie oi-ganisni, but their conii)osition varies in such a way thattheii- 

 influence ma.y be either the same or in certain respects different. 



The distribution of the chromosomes in the two meiotic divisions L'^ 

 shown diagrammatically in Fig. 75. The remarkable alterations in form 

 undergone by the chromosomes during the successive stages are not 

 represented here; these will be described in the following section. A 

 diploid number of 6 is arbitrarily chosen. The members of the two gen- 

 omes are distinguished by shading and bv large and small letters. Dots 

 in the uppermost nuclei indicate location of kinetochores. 



Referring to the first column in the diagram, we see that the six 

 chromosomes are arranged in no particular order in the nucleus. In the 

 prophase of the first meiotic division the tw-o chromosomes of each 

 homologous pair approach each other and become very intimately 

 associated, but they do not actually fuse; this is sy^iapsis. The chromo- 

 somes are now in the bivalent condition. Xot long afterw^ard each of the 

 members of the s.ynapsed pair becomes visibl.y double apparently by 

 splitting; the bivalents now have the form of tetrads, the four members of 

 each tetrad being chromatids. At the end of prophase / the nucleus still 

 has all the chromosomes, but they appear as the monoploid number of 

 tetrads. 



In metaphasc / the tetrads are arranged in the equator of the spindle, 

 and in the anaphase each of them separates into tw^o dyads, or pairs of 

 chromatids, which pass to the two poles. In the diagram the arrangement 

 at metaphase is such that sister chromatids (those formed by the recent 

 splitting) pass to the same pole; hence it is the members previously 

 brought together by synapsis which separate here, and this form of 

 separation is called disjunction. 



Each of the resulting nuclei then undergoes a second meiotic division. 

 Here the two chromatids composing each dyad separate equationally 

 (along the recent split) and paiss poleward. Meiosis thus results in a 

 quartet of nuclei in each of w^hich there is one complete genome with three 

 members. The four chromatids of each tetrad now lie in four different 

 nuclei. 



Several features of the process thus far described should be carefully 

 noted, for it will be necessary to qualify present statements after other 

 modes of distribution have been considered. (1) As illustrated in column 

 1, the first division is disjunctional and the second equational for all the 

 chromosomes. (2) In the resulting quartet of nuclei there are genomes of 

 two kinds with respect to the derivation of their members from the two 

 original genomes. (3) The arrangement shown at metaphase / is only 

 one of several possible ways in which the chromosomes could be arranged 

 and still have all the separation disjunctional at anaphase. With three 



