THE POLYPLOID SERIES IN OSMUNDA 



Since the spiral forms independently in each chromosome the paired partners 

 lose contact with one another as prophase advances, except at a few places at 

 which an especially intimate relation has been established. Such places are termed 

 chiasmata and one, two or three may occur in any chromosome pair (in other 

 organisms with longer chromosomes the number may be greater) in positions which 

 are to some extent determined at random. The shapes of paired chromosomes are 

 affected by this, although the nature of chiasmata are not otherwise important for 

 our present purpose and need not therefore be discussed. If a pair of chromosomes 

 remains joined by a single chiasma it will look like a rod, V or X, according to whether 

 the chiasma is median in position or terminal. If there are two chiasmata a pair of 



mm iPk "^ 



Fig. 21. The equatorial plate stage of the first meiotic division in diploid Osmunda showing 22 pairs 

 of chromosomes in which spiral structure has been revealed by ammonia treatment before fixation. 

 The formation of the spiral explains most, though not all, of the apparent changes of size and 

 shape which a chromosome experiences during prophase. Fresh acetocarmine. x 2000. 



chromosomes will resemble an O or an a; if there are three it will look like a figure of 

 eight. Several of these shapes can be seen in Fig. 22 a, which represents the end of the 

 first meiotic prophase at the stage known as diakinesis. After this the paired chromo- 

 somes assemble on the spindle and then separate to opposite poles. 



The details of the later stages of meiosis need not at the moment concern us, and it 

 is perhaps sufficient to refer to the anaphase of the second division (Fig. 22b) in which 

 the reduced (haploid or monoploid) number of 22 single chromosomes is very clearly 

 displayed. 



When more than two homologous sets of chromosomes are present both meiotic 

 divisions are affected in a characteristic way. If there are three homologues of every 

 chromosome they will attempt to unite in threes, an attempt which is not always 



35 . 3-' 



