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THE AMERICAN NATURALIST [Vol.XLVI 



forms, directly from a cliromatin-network. They arise 

 from massive bodies, each of which resolves itself into a 

 closely convoluted thread which then uncoils before con- 

 jugation takes place. Why should chromosomes that are 

 already formed as massive bodies delay their division 

 or conjugation until so remarkable a redistribution of 

 their substance has taken place? It is not a necessary 

 condition of conjugation, as is proved by the case of both 

 the sex-chromosomes and the m-chromosomes, which do 

 in fact conjugate in the massive condition. All the facts 

 become intelligible in the light of Eoux's hypothesis that 

 the formation of the spireme-threads effects a linear 

 alignment of different constituents in preparation either 

 for division or for a definite type of association in pairs 

 in synapsis. 



One of the most interesting applications of this view to 

 genetic phenomena is that suggested by Janssens in his 

 theory of the "chiasmatype," which has recently been 

 applied by Morgan to the explanation of coupling and 

 repulsion. In the twisting together of the spireme 

 threads, either in synapsis or at a succeeding stage, and 

 the subsequent secondary splitting of the thread in one 

 plane is provided a very simple mechanical basis, both 

 for the free interchange of factors between the homol- 

 ogous chromosomes and for the phenomena of coupling 

 or repulsion, which are otherwise so difficult to compre- 

 hend. I do not maintain that this particular interpreta- 

 tion, or the more general one of Eoux, is demonstrably 

 true, or that no other explanation can be found. I only 

 hold that they are legitimate conceptions which may be 

 tested by observation and experiment, and which must be 

 fully reckoned with as intelligible interpretations of the 

 facts before we can set these facts aside as utterly mys- 

 terious or as a meaningless by-play. 



3. I would lastly recall the experimental proof by 

 Boveri that the chromosomes differ among themselves in 

 their physiological relation to development, and the cor- 

 responding cytological fact that they differ among them- 



