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THE AMERICAN NATURALIST [Vol. LIV 



tions there is nothing in the history of these crosses, as 

 thus far made known, to suggest an earlier process of 

 torsion, chiasma-formation, and recombination. They 

 indicate rather that the cleavage of such a tetrad straight 

 through its two clefts involves simply one reduction- 

 division and one equation-division (Fig. 6 D-E). 



Robertson has pointed out that many of those appear- 

 ances in the prophase- and metaphase-tetrads on which 

 Janssens's theory was originally based are susceptible of 

 a much simpler explanation than is offered by the chias- 

 matype-theory, namely, that they are a result of "mis- 

 fortune in the prophase, ' ' due to secondary displace- 

 ments of torsions at this time. Experiments with clay 

 models have convinced me that this point is well taken, 

 in respect to some at least of these appearances. It should 

 also be clear from the foregoing discussion that condi- 

 tions resulting from the persistence of the so-called two- 

 strand chiasma in the metaphase-figures are readily ex- 

 plicable without the assumption of an earlier process of 

 chiasmatypy. 



In this brief review and critique of the cytological as- 

 pects of the question, I have not intended to take up an 

 attitude of opposition towards the chaismatype-theory 

 considered as an explanatory principle in genetics. On 

 the contrary, I am not able to escape the conviction that 

 somewhere in the course of meiosis some such process 

 must take place as is postulated by Janssens and by Mor- 

 gan and his co-workers, though I must admit that this 

 opinion rests less on cytological evidence than on genetic. 

 I have wished only to discuss the possibilities of the ex- 

 isting cytological situation and to offer a counsel of cau- 

 tion in respect to the chiasmatype-theory in so far as it , 

 is based on conditions seen in the later stages of meiosis. 

 This means no lack of appreciation for Janssens's bril- 

 liant and fruitful work, which has opened up so remark- 

 able a new field of inquiry. But a theory of such funda- 

 mental importance calls for critical treatment, and on its 



