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THE AMERICAN NATURALIST [Vol. LIV 



think, that the chiasmatypy has taken place during a 

 strepsinema stage prior to the straight, longitudinally 

 divided threads from which the rings arise (Figs. 4 B, C, 

 at the left). If now, for the sake of argument, we accept 

 these assumptions, how does it come to pass that the sub- 

 sequent opening out of the rings exactly fits with the re- 

 combination-phenomena that have previously occurred 

 in the tetrad? Morgan. has already supplied an answer 

 to this in the ingenious suggestion that the mode of sep- 

 aration of the threads may he determined by their nature 

 —i. e., that the paternal and maternal threads (or por- 

 tions of threads) may always be the first to separate, 

 however they may lie in the tetrad. 6 This is an impor- 

 tant addition which makes the whole series of assump- 

 tions logically complete. 



All this constitutes a somewhat complicated train of 

 reasoning; nevertheless, if it be granted, it provides for- 

 mally an escape from the seeming contradiction and 

 leaves the chiasmatype-theory intact. The point, how- 

 ever, that I wish to emphasize is that we have now passed 

 over into a realm of hypothesis and logical construction, 

 based it is true on a vast assemblage of data of the high- 

 est importance, but derived from genetic experiment 

 rather than from cytological observation. No observer, 

 so far as I know, has yet seen a process of true crossing- 

 over (recombination) by means of torsion, chiasma-for- 

 mation, fusion, and secondary splitting apart. That such 

 a process takes place at all remains thus far an inference 

 based on the presence of a continuous two-strand chiasma 

 in later stages of meiosis and on certain resulting ap- 

 pearances in the late prophase- and metaphase-tetrads. 

 But as shown above, precisely the same appearance of a 

 two-strand chiasma is given by a process in which no tor- 

 sion need be involved. Both Wenrich and Eobcrtson 

 have urged this fact against Janssens's interpretation; 

 and I am fully in agreement with them so far as the later 

 stages of meiosis are concerned. It may nevertheless be 

 pointed out that both these observers have figured stages 



« '19, pp. 101-104. 



