268 



INTRODUCTION TO CYTOLOGY 



chiasmata at diplonema and about 10 at diakinesis. In Rosa the mean 

 number of chiasmata per tetrad is 2.66 at late diplonema and 1.53 at 

 metaphase /. As a general rule each tetrad retains at least one chiasma 

 or a terminal association through to the metaphase, indeed, it is believed 

 by some observers that the continued association of the dyads at meta- 

 phase is due primarily to the presence of retained chiasmata rather than to 

 any mutual attraction existing between them at this time (Darlington). ^^ 



Fig. 157. — Diagram of a tetrad with one chiasma and chromatid exchange as inter- 

 preted on the two-plane theory {A) and the one-plane theory {B). The chromatids of 

 the two conjugated chromosomes are shown in solid lines and broken lines, respectively, 

 and one end of each is marked with a knob. Spindle-attachment regions indicated by 

 stippling. 



With regard to the origin of the chiasma, there are two principal 

 interpretations which for convenience may be called the "two-plane 

 theory" and the "one-plane theory" (Fig. 157). 



According to the two-plane theory, ^^ when the four chromatids seen in 

 the pachynema stage open out two by two to give the diplonema condi- 



23 The statistical statements in this paragraph involve the assumption that the 

 nodes and terminal associations observed in the tetrads actually represent chiasmata. 

 In many instances this interpretation is probably open to question. In view of 

 genetic evidence and the behavior of small chromosomes, particular!}' in individuals 

 with no crossing-over, it seems especially improbable that strictly terminal associa- 

 tions at metaphase represent chiasmata (Sax, 1930rf; Reuter, 1930; Belling, 19316). 



^_* Granata (1910), McClung (1914, 1924, 1927a6), Wenrich (1916), Robertson 

 (1916), Wilson (1925), Seller (1926), B61af (1928), Sax (1930c, 1932), Newton (1927), 

 Newton and Darlington (1929), Darlington (1929c; see, however, our footnote 26), 

 Babcock and J. Clausen (1929). 



