MEIOSIS 275 



another region; or, if the two are synapsed throughout their length, the 

 third may remain free altogether. The genetic evidence in triploid 

 Drosophila also indicates such behavior. These phenomena suggest 

 that the "synaptic force" operating at any region in the homologues is 

 somehow "exhausted" when two members are intimately synapsed, so 

 that the third member cannot establish synaptic union with them at 

 this region. Chemical and electrical analogies naturally come to mind, 

 but nothing is known about the precise nature of the forces concerned. 



That synaptic "attraction" exists primarily between pairs of elements 

 is suggested further by the opening out of ordinary bivalent tetrads at the 

 diplonema stage. After the longitudinal doubleness of each synaptic 

 mate becomes complete and obvious, whatever the time of its inception, 

 the four chromatids "fall apart" into pairs except at chiasmata, as if the 

 forces tending to hold the threads together could now operate between 

 any two of them but not between all four. It is largely upon phenomena 

 of this character that certain current theories of the origin and causes of 

 meiosis have been based. 



The suggestive hypothesis advanced by Darlington (see especially 

 193 Ic^ and 1932a) may be stated briefly as follows. During their pro- 

 phasic contraction, chromosomal threads normally show a strong tend- 

 ency to be associated in pairs. When the contraction begins in somatic 

 mitosis the threads are already double as a result of splitting,^" so that 

 this tendency is already satisfied. In the meiotic prophase, on the con- 

 trary, the contraction begins before the splitting; hence the tendency to 

 be double is satisfied by the synapsis of the threads in pairs. After the 

 paired threads become split in the late pachynema stage, this pairing 

 becomes superfluous and the tetrad opens out to give the diplonema con- 

 dition, the split threads remaining associated only at chiasmata until 

 anaphase. Meiosis is thus conceived to be due to a precocious prophasic 

 contraction of the chromosomes, this precocity being in some way asso- 

 ciated with unusual features of the preceding interphase. 



An alternative hypothesis is that advanced by Huskins (1932a). This 

 author accepts the evidence that the chromosomal threads may split one 

 mitotic cycle in advance of their separation in somatic divisions and 

 therefore contends that the event which initiates the meiotic process is 

 the suppression or retardation of splitting during the last premeiotic 

 division, rather than the precocious contraction in the meiotic prophase. 

 As a result of such suppression, which is probably due to the peculiar 

 physiological state of the cells at this period, the leptonema threads of 

 the meiotic prophase are single until after the contraction and synapsis 

 begin and must therefore undergo their splitting in the pachynema stage. ^^ 



^^ Darlington assumes the splitting to take place in the metabolic stage and rejects 

 the evidence for its earlier occurrence. 



^^ As already pointed out, Huskins finds the metaphase tetrads of Trillium to have 

 four double chromonemata. Hence the synapsing threads are thought to split twice 



