No. 632] CHIASMATTPE AND CROSSING OVER 211 



purely cytological side too much has sometimes been 

 taken for granted by writers on genetics. It is, I think, 

 highly probable that the cytological mechanism of cross- 

 ing-over must be sought in some process of torsion and 

 recombination in the earlier stages of meiosis— perhaps 

 during the synaptic phase of slightly later— and that this 

 process may leave no visible trace in the resulting spi- 

 reme-threads. To accept this, of course, would mean that 

 such conventionalized diagrams as those here offered 

 (Figs. 3, 5, etc.) should be so modified as to indicate ox- 

 changes which have earlier taken place between the syn- 

 aptic mates. It must be said, on the other hand, that the 

 actual evidence of torsion during the process of parasyn- 

 apsis is still very inadequate and receives no support 

 from some of the most careful recent work. One can not 

 avoid a suspicion that some internal process of torsion 

 (or of rotation, as conjectured by Correns) may take 

 place in the early pachytene before the duality of the 

 diplotene becomes externally visible. Conjecture con- 

 cerning all this will however be less fruitful than 

 further cytological analysis. The truth is that for 

 the time being genetic development of the chromosome- 

 theory has far outrun the cytological. We are in no posi- 

 tion to predict when the plodding progress of cytology 

 may be able to close the gap % : nevertheless we have every 

 reason to hope that the physical mechanism of the recom- 

 bination-phenomena may in the end prove to be accessible 

 to decisive cytological demonstration. 



The Spiral Looping of the Chromosomes and the 

 Theory of Crossing Over 



T. H. MORGAN 



In his two recent papers Janssens calls attention to 

 certain details relating to the application of the findings 

 of cytology to the interchanges between homologous sets 



situation as it is uvnrra ll> undi-rstuud to-day, and calls 



