Reduction 65 



At the beginning of meiosis the primary spermatocytes and 

 primary oocytes of the above animal have 16 chromosomes. At 

 the end of the first division, the secondary spermatocytes and 

 the secondary oocytes have only 8. Since the nmiiber is re- 

 duced half, this division is often referred to as the reduction 

 division. The reduction division reduces the number of chromo- 

 somes and centromeres. In the second meiotic division, the chro- 

 matids of each chromosome separate from one another. Because 

 of the "split'' of the centromeres, there is no further reduction 

 of the number of centromeres or of chromosomes in each daughter 

 cell. It is not, therefore, a reductional division, but is often 

 called the equation division because of the equal separation of 

 sister chromatids to the daughter nuclei. 



Usually the first division is reductional and the second equa- 

 tional for the number of effective centromeres and for the 7ium- 

 ber of chromosomes. Because of breaks in the chromatids and 

 the formation of new combinations of segments of chromatids at 

 pachytene which result if chiasmata occur at diplotene, reduction 

 is true only in a quantitative sense and is not true qualitatively 

 for the whole of all the chromatids. If there were no exchanges 

 of chromatids and no chiasmata (assuming that the chromosomes 

 were still paired), the entire homologues would separate reduc- 

 tionally in a cjualitative sense at first anaphase and equationally 

 at second anaphase. Normally, however, one or more chias- 

 mata are formed. In a chromosome having one long and one 

 short arm, let us assume that one chiasma can form in the long 

 arm but that the other arm is too short for a chiasma. The 

 result is a chromosome passing to one pole at first anaphase 

 which is composed of one normal chromatid plus a sister chro- 

 matid which is normal for the short arm and the part of the 

 long arm nearest the centromere (the proximal part) but has a 

 distal piece of a homologous chromatid. Similarly, the chromo- 

 some passing to the other pole is normal except for the corre- 

 sponding distal segment. As a result, the first anaphase is reduc- 

 tional for the centromere, short arm, and proximal piece of the 

 long arm, but is equational for the distal end of the long arm 

 of one chromatid of each chromosome. Correspondingly, the sec- 

 ond division is equational for the short arm and proximal part of 

 the long arm but reductional for the distal portions. If more 

 than one chiasma is present, the chromatids are more complex. 



