58 Reproduction and Meiosis 



so easily observed at pachytene because the threads lie close 

 together, but at diplotene as the result of the repulsion and con- 

 sequent opening out of the threads, the four-strand nature is 

 easily seen. The diplotene chromatids are long and thin but, as 

 diplotene progresses, they contract greatly and become much 

 thicker. This contraction is due to the coiling or spiralization 

 of the long, thin threads that were present originally. At the 

 same time, they rotate in such a way that in a bivalent with 

 several chiasmata the successive loops lie at right angles to one 

 another, whereas if only one chiasma is present, the arms rotate 

 through 180°. As at mitosis, a matrix which stains very heavily 

 begins to collect around the threads so that the internal struc- 

 ture of the chromosome is not easily visible late in this stage. 

 The repulsion which starts with the beginning of diplotene con- 

 tinues and is often strong enough to cause the chromatids to slide 

 along one another so that the chiasmata appear to move towards 

 the ends. This terminalization begins towards the end of diplo- 

 tene and may continue through the next stage and up to meta- 

 phase. Chiasmata may terminalize completely or only partially. 

 Terminalization is, in general, greatest in small chromosomes and 

 least in large ones, although the degree of terminalization also 

 seems partly a characteristic of certain species. 



The chromosomes pass gradually from diplotene to the last 

 stage of the prophase, diakinesis. At this stage, the bivalents 

 are very short and thick and are quite deeply stained, and the 

 two chromatids of each chromosome are close to one another, with 

 the result that the identity of the individual chromatids is usually 

 lost except possibly at the ends when the chiasmata are not 

 completely terminalized. During this stage, the spiralization of 

 the chromatids may continue, causing them to become somewhat 

 shorter than at the beginning of diakinesis, and terminalization 

 may also continue if it had not been completed in diplotene. The 

 bivalents tend to repel each other during diakinesis. They move 

 to the periphery of the nucleus just inside the nuclear membrane 

 and are frequently arranged so that each is as far away from 

 every other one as it can get, although this last feature seems 

 more noticeable in small than in large nuclei. The nucleolus 

 disappears during diakinesis, and this stage is terminated by the 

 disappearance of the nuclear membrane. 



