260 INTRODUCTION TO CYTOLOGY 



diplonema stage the threads become progressively shorter^" and pass 

 gradually into the stage known as diakinesis, toward the end of which 

 they become compact bodies lying well scattered throughout the nucleus. 



At diakinesis the tetrads differ in several respects from those seen 

 earlier in the prophase. Their length has become greatly reduced. In 

 certain cases it has been found that a tetrad may be only one-fifteenth or 

 one-tenth as long as the leptonema and early pachynema threads from 

 which it is derived. This shortening, from the time it begins in the early 

 prophase, involves a number of characteristic alterations which are, 

 however, not precisely comparable in all organisms. As the pachynema 

 threads shorten, their small ("ultimate") chromomeres may group into 

 larger ("secondary") ones (c/. p. 143). Later on, notably as the diplo- 

 nema stage passes into diakinesis, the matrix becomes conspicuous, giving 

 the tetrad a smoother contour. At the same time the chromonemata 

 become more uniform in thickness and may form rather regular spirals 

 within the matrix. "^^ In rye the chromosome shortens about one-third 

 between diplonema and metaphase, but the chromonemata retain their 

 original length by coiling; during metaphase the chromonemata shorten 

 about one-third while the chromosome length remains unchanged. In 

 other organisms, notably maize and grasshoppers, the chromonemata at 

 diakinesis do not form very conspicuous spirals, the chromosome exhibit- 

 ing a structure resembling that in the pachynema stage more than would 

 be thought possible after so much shortening. This raises interesting 

 questions regarding the transfer of materials to and from the chromosomes 

 during these stages. 



The various peculiar shapes assumed by the tetrads at diakinesis are 

 due mainly to the position of their spindle-attachment regions, which 

 look as if they were repelling each other, and to the number and position 

 of their chiasmata. A single interstitial (non-terminal) chiasma gives 

 the tetrad the form of an X ; a terminal chiasma or association gives a V ; 

 two terminal ones give an ; other arrangements of chiasmata give other 

 shapes, as can be readily seen in Figs. 150 to 156. Since long chrome- 

 somes tend to have more chiasmata than short ones, the small tetrads of 

 a group are in general simpler in form than the large ones. Moreover, 

 the number of chiasmata per chromosome tends in many organisms to be 

 reduced in passing from diplonema to diakinesis, a fact whose theoretical 

 importance will be touched upon in a later section. Although the type 

 of tetrad which develops is thus largely due to the chiasmata formed by 

 the chromonemata, the matrix contributes much to its general appearance. 

 In some tetrads, notably short ones in animal meiocytes, the matrix is 



1" It is here that a "second contraction" stage is often described in accounts of 

 meiosis. 



" E.g., Babcock and J. Clausen (1929) on Crepis, Sax (1930c) on Secale, Taylor 

 (1931) on Gasteria, and Nebel (1932) on Tradescantia. 



