106 FUNDAMENTALS OF CYTOLOGY 



separate disjunctionally in I and equationally in // (prereduction), 

 while the distal portion beyond the chiasma separates equationally in I 

 and disjunctionally in II (postreduction). The proximal portions of the 

 chromatids eventually lie in the four (quartet nuclei precisely as they do 

 in column 1. The distal portions also lie in the four nuclei. The 

 complication introduced by crossing over appears when the relative 

 positions occupied by the two portions are considered. 



In the few cases supported by adequate cytological evidence (visibly 

 unlike homologues; data on chiasmata) and genetical evidence (distribu- 

 tion of genetical factors), it appears that the four chromatids behave as 

 described above: the regions near the kinetochores disjoin in /, while 

 equational division in I occurs only in certain regions determined by 

 crossovers. If these latter regions show visible differences in the two 

 homologues, their postreduction is evident, but it does not follow that 

 the whole tetrad separates in this way. Hence reduction in the strict 

 sense (disjunction) does not take place in all portions of the complement 

 or even in all portions* of the same tetrad at the same division. It 

 is only after both meiotic divisions have been carried through that 

 chromosome reduction is complete. It is only then that all disjunction 

 is finished, and only then does each nucleus have the reduced number of 

 single chromosomes. 



The points brought out in this section may now be summarized. In 

 meiosis each chromosome enters into synapsis with its homologue and also 

 splits longitudinally, giving thus a tetrad composed, of four chromatids. 

 The four chromatids of every tetrad are distributed iri two divisions to the four 

 nuclei formed at the close of meiosis. Each chromosome {or portion of a 

 chromosome) is disjoined from its homologue {reduction) in one of the 

 divisions and divided equationally in the other. It is probable that dis- 

 junction in the first division is the rule for the kinetochores and near-by 

 regions, the second divisio7i therefore being equational for these regions. This 

 order can be reversed in other regions when crossovers occur. 



Each of the four nuclei of the resulting quartet contains a single genome 

 composed of members from one or both of the genomes of the original diploid 

 complement. The four nuclei are qualitatively alike or unlike depending 

 upon the amount of difference between homologous chromosomes in the 

 original complement. Every kind of chromosomal unit is present singly 

 instead of in duplicate in each nucleus. 



Detailed Account of the Phases of Meiosis. — Most of the significant 

 features peculiar to meiosis are found in the prophase of the first division ; 

 when these are understood the subsequent stages present few difficulties. 

 The phases of the entire process will now be described in order (see Fig. 

 76). It is to be remembered that the details of meiotic chromosome 

 behavior vary a good deal in different organisms and that the purpose of 



