218 THE CONTINUITY OF THE RACE 



are termed bivalents, and the number of bivalents is, of course, just half 

 the number of chromosomes to be seen in mitosis. 



Following and overlapping with the zygotene substage is a third sub- 

 stage of prophase called pachytene, in which the chromosomes become 

 much shorter and thicker by forming tight spiral coils, much as in the 

 prophase of mitosis. 



The fourth and final substage of the meiotic prophase that we shall 

 recognize is known as diplotene. In this it becomes apparent that each 

 bivalent consists of four strands, each of the homologues now at last 

 showing its two sister chromatids. Just when the meiotic chromosomes 

 actually split into sister chromatids is a matter of conflicting evidence 

 and views (the weight of the evidence indicating some time during 

 pachytene), but there is no doubt that each bivalent consists of four 

 strands at the beginning of diplotene. Now the attraction between 

 homologous chromosomes ends and is succeeded by a marked repulsion 

 between them. 1 



As the homologous pairs separate it becomes evident that still another 

 process has been involved. One (or both) of the chromatids of one of the 

 homologues has "crossed over" with one (or both) of the chromatids of 

 the other. These crossovers tend to hold the separating chromosomes 

 together, so that although the regions between and beyond the crossover 

 points continue to widen, the homologous pairs are held together until 

 their final separation at anaphase. 



The crossovers or chiasmata are of very great importance, in that each 

 one is the result of the breaking of two homologous {not sister) chromatids, 

 with an exchange of comparable parts. The part or parts of one chromatid 

 unite with the complementary parts of its homologue by an end-to-end 

 junction at the point of breakage. In a single crossover between homolo- 

 gous strands those portions of each chromatid between the break and the 

 free end (the end lacking the centromere) are interchanged. When there 

 are two or more crossovers, it is the portions between two adjacent 

 crossovers that are exchanged. 



There is much evidence that the actual breaking and reuniting of the 

 chromatids takes place in pachytene, soon after the members of each 

 bivalent have split to form a four-strand structure and before the chias- 

 mata are made visible by the moving apart of the homologous strands in 

 diplotene. It should be emphasized that normally the exchange of chro- 



1 A very plausible theory advanced by Darlington holds that in the absence of 

 chromatid formation in early prophases of meiosis, the attraction that in mitosis 

 holds the sister chromatids in close juxtaposition is expended in drawing the homolo- 

 gous chromosomes together. Then, when the meiotic chromosomes do finally split into 

 chromatids, the attraction force is satisfied by the sister chromatids, and a repulsion 

 is built up between the homologues. 



