DIRECTION OF COILING 487 



the case in Tradescantia. In Trillium and Fritillaria, on the other 

 hand, the coiling of the chromatids seems to be regularly separate 

 after fixation (Huskins and Smith, 1935, D. 1935). 



During the interphase between the two divisions we find relic 

 spirals gradually uncoiling as at mitosis. Further, since the sister 

 chromatids of each chromosome have separate coils we might 

 suppose that these coils were embodied in the internal coils of the 

 second metaphase. But this seems unlikely, for we find that owing 

 to the shortness of the interval the relic coils may survive until 

 metaphase, superimposed on the internal coils as they are in the 

 prophase of mitosis (D. 1936 b). 



The coiling system at second metaphase usually consists of a 

 single internal spiral as at mitosis. The degree of contraction at 

 this stage is, however, variable in many organisms {e.g., Gasteria, 

 Podophyllum, Fritillaria) and it is not surprising to find that major 

 as well as minor spirals are found regularly in Sagittaria and occa- 

 sionally in Lilium longiflonim. 



A fixation that contracts the staining thread, obscuring the minor 

 spiral, is especially favourable for showing the direction of the 

 major spiral at first metaphase and anaphase. In bivalents of 

 Tradescantia and Rhceo with only terminal chiasmata the simplest 

 rules are found. The direction is constant for all chromosome-arms, 

 and since the arms are jointly coiled the paired chromatids neces- 

 sarily have the same direction of coiling. Pairing arms often 

 have opposite directions of coiling however (Fig. 137). Where, 

 therefore, interstitial chiasmata are found at metaphase in Trade- 

 scantia and Trillium, changes of direction occur not only at the 

 centromere but in chromosome arms. These changes perhaps 

 coincide with the interstitial chiasmata (Sax and Humphrey, 1934) 

 and result from crossing-over— presumably between chromosomes 

 with opposite directions of coiling (Huskins and Smith, 1935 ; 

 Matsuura, 1935). Changes of direction are probably to be inferred 

 from the observation of constrictions in two of the four chromatids 

 at anaphase in Stenohothrus (Fig. 36A ; cf. Belar, 1928). They are 

 not, however, inherent in crossing-over, for acentric chromatids, 

 which must always result from crossing-over, have been found with 

 no changes of direction (S. G. Smith, 1935 ; Upcott, unptib.). 



