98 



MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



The first evidence of further change is seen in the separation of the 

 partner chromosomes making up the double thread. It is then 

 found that each of the new threads is double. Each chromosome 

 has divided into two chromatids. This stage is diplotene. 



The evidence of the best material (Janssens, 1924 ; Newton, 

 1927 ; Belar, 1928 b) is conclusive that the separating threads are 

 double. In Fritillaria where pairing begins near the centromere, 

 separation begins in the same region. In Stenobothrus the splits 

 appear in the paired threads at several points and extend until they 



Fig. 29. — A. Pachytene in Fritillaria imperialis ; pairing incomplete 

 near the attachments of two pairs of chromosomes to the 

 nucleolus {cj. Plate XV). B. Beginning of diplotene in F . obliqua, 

 separation showing in two pairs at the centromere. Lg, relational 

 coiling ; Rg, relic coiling, x 2000 (D., 1935 b). 



meet. It is then found that the splits separate different pairs of 

 chromatids, for, at the point of meeting, the pairs change partners, 

 giving, from one point of view, the appearance of a cross ; hence the 

 name chiasma is applied to the occurrence of this exchange (Janssens, 

 1909, 1924 ; Belling, 1928 c ; D., 1930 b, 1931 c). 



Chiasmata are formed as a rule in all paired chromosomes at 

 this stage. But, as we shall see later, short chromosomes which 

 have been paired at pachytene may in some cases be presumed to 

 fall apart at this stage without the formation of chiasmata. They 

 become " unpaired chromosomes." 



Chiasmata probably always arise in the first instance interstitially, 



