270 INTRODUCTION TO CYTOLOGY 



strains upon two of the chromatids, especially those crossed and in con- 

 tact, that they would find it easier to break at the point of greatest strain 

 than to separate unbroken (A in Figs. 157 and 158). In this way they 

 would exchange equivalent portions. ^^ Such a process would also result 

 in genetic recombination as well as in a reduction in the number of 

 chiasmata. It has been suggested further that such an exchange between 

 chromatids might be caused by the resistance of chiasmata to the forces 

 which draw the tetrads apart into dyads at anaphase. 



The one-plane theory-^ is essentially the "incomplete chiasmatypy" of 

 Janssens. According to this view, the openings between the chromatids 

 in the diplonema stage are all in the synaptic plane: the portions of two 

 chromatids running closely parallel in any region when the tetrad first 

 opens are always sisters {i.e., two formed by splitting of one) (Figs. 

 157, B; 158, B). Hence, in order to account for the characteristic 

 arrangement of the four chromatids at the chiasma, it is necessary to 

 assume that an exchange of portions between two of them must have 

 occurred at this point. In other words, chiasmata regularly indicate 

 crossing-over. How such an exchange could occur in the pachynema 

 tetrad before its opening out is an unsolved problem." 



To account for the reduction in the number of chiasmata observed 

 between diplonema and late diakinesis, Darlington postulated a process 

 called ''terminalization."-^ This is a movement of the chiasmata away 

 from the spindle-attachment region toward the ends of the tetrad, the 

 four threads closing ahead of the moving chiasma and opening in the 

 other plane behind it (Fig. 158, B). The single terminal association 

 remaining in many tetrads at late diakinesis is accordingly regarded as a 

 "terminal chiasma, " that is, one which has reached the end of the tetrad. 



2^ According to the hypothesis of Sax (1930c) a partial twisting of the tetrad bi'ings 

 two of its chromatids into close contact, the strains set up b}^ further twisting or 

 unequal contraction causing a break at the point of contact. The same result could 

 be accomplished by the forces opening the tetrad without twisting and contact, pro- 

 vided the paired threads were meanwhile held strongly in synapsis close to the chiasma 

 on either side, for there is evidence that broken ends of chromosomes have a tendency 

 to reunite (McClintock, 1932c). 



25 Janssens (1909, 1924), Belling (19276 et seq.), Darlington (1930c et seq.; cf. 

 our footnote 24, p. 268), Maeda (1930a6). 



27 According to the hypothesis of BeUing (19316, 1933), new longitudinal connecting 

 strands develop between successive daughter chromomeres formed by division in the 

 pachynema stage, such strands forming at half-twists in the threads being so arranged 

 that they join portions of different chromatids. If the two strands forming the cross 

 lie in the same plane {i.e., on the same side of the tetrad considered as a regular 

 bundle of four chromatids), the chiasma is said to be "direct"; whereas, if they lie 

 in different planes (on different sides of the tetrad), it is said to be "oblique." The 

 chromatids "crossed" at the chiasma are unchanged, while those running straight are 

 the exchanged ones. 



28 See especially Darlington (19296) on Tradescantia and (1931a) on Primula; also 

 (1931d, 1932a), where observations of other investigators are interpreted on this basis. 



