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University of California Publications in Agricultural Sciences [Vol. 2 



The chiasmas must have been formed earlier but are not seen 

 clearly before the two homologues uncoil. They mifjht have been 

 formed, as Belling suggests (1928a, pp. 28.8-290), by coincident 

 breaks in two of the chromatids of the homologues and subsequent 

 attachment of the wrong ends of the broken chromatids as text figure 

 1, a, h, shows. When the homologous (double) chromosomes are sep- 

 arated in heterotype meta-anaphase the rearranged chromonemata 

 probably are pulled out of the chromosomes to which they originally 

 belonged, as figure 1, c, d, indicates. In these figures no attention has 

 been paid to the complication caused by the spiral twisting of the 

 chromatids around one another or to the chromomeres. 



Fig. 1. Diagram illustrating the way crossing over might take place between 

 two homologous chromosomes in the four-strand stage and chiasmas might be 

 formed in accordance with the observations on Crepis. 



a. The four strands twisted around each other in diplophase. A coincident 

 break is shown in two chromatids from different homologues; the "matrix," in 

 which the chromatids later on appear embodied, does not show in this stage, 

 therefore the connection is more close. 



b. Diaphase. The outline of the chromosomes shows up. An X-chiasma has 

 been formed by wrong attachment of the two chromatids at the breaking-place. 



c. Heterotypic metaphase; one of the two partners has turned around, a cross 

 has been formed by pulling, but the chromosomes still have retained their indi- 

 viduality. * 



d. Heterotypic anaphase; the two chromatids with exchanged sections have 

 been pulled out from their respective chromosomes and are enclosed within the 

 matrix of the anaphasic chromosomes. * 



e. Tetrad stage; each of the four different chromatids has split longitudinally. 



As will be understood, in all gemini in w^hich chiasmas occur the 

 X-crossed chromatids run from one chromosome to another. Which 

 two of the four chromatids will go to either pole during anaphase will 

 depend upon which two are together in the ends of the chromosomes 



