CHARACTER OF MEIOSIS Sy 



of diploid species of Fritillaria. Alternative nomenclature will be 

 found in the glossary. 



I. First Division, {a) Leptotene. The chromosomes appear in 

 the nucleus in their diploid number. They are single threads, not 

 double as in mitosis. Each has an uneven granular structure which 

 gives it the appearance of a string of unequal beads, unequally 

 strung together. The beads are the chromomeres. The threads are 

 disposed evenly in the nucleus. 



(b) Zygotene. The chromosomes come together in pairs, corre- 

 sponding chromomeres lying side by side. They usually come into 

 contact first near the centromere (and this body can be picked out 

 by the greater staining capacity of the chromomeres on either side 

 of it). But association often begins at other places independently. 

 It extends along the chromosomes until the whole complement are 

 present as double or bivalent threads in the haploid number. 



(c) Pachytene. The paired threads of each bivalent coil round 

 one another and their proximal chromomeres increase in size still 

 further. The chromosomes thus show differential condensation in 

 different parts. 



{d) Early Diplotene. The chromosomes fall apart and at the 

 same time each is seen to be double, consisting of two chromatids, 

 which remain in close association like that of the chromosomes at 

 pachytene. The chromosomes separate completely except at 

 various pomts along their length, where their chromatids exchange 

 partners. These cross-shaped exchanges are called chiasmata and 

 number one, two, or more in each bivalent. They are due to 

 crossing-over between two chromatids of the partner chromosomes, 

 i.e., the two chromatids have broken between corresponding pairs 

 of chromomeres and rejoined across to give two new combinations. 

 The chromosomes now begin their spiralisation. 



{e) Late Diplotene. Spiralisation continues, the chromosomes 

 straighten and successive loops between chiasmata come to lie at 

 right angles. 



(/) Diakinesis. Spiralisation reaches a maximum and the 

 chromosomes are shorter than at a somatic mitosis. They continue 

 to be evenly disposed throughout the nucleus. The nucleoli, which 

 have been attached to their organisers on the chromosomes, disappear. 



