No. 632] C HI A SMA TYPE AND CROSSING OVER 217 



threads may be thought of as separating without assum- 

 ing such a strictly symmetrical form as Janssens's new 

 diagram indicates, or in other words the separation of 

 the chromosomes may take place as Janssens described 

 it in Batracoseps. The suggestion that I made to account 

 for the appearance of rings in different planes was made 

 to meet an objection raised by Kobertson and by Wen- 

 rich, namely, that the crossed threads (the chiasma 

 threads) do not mean that crossing over has taken place 

 in that region. They point out that the crossed threads 

 may mean no more than that a not-twisted tetrad has 

 opened out in different planes in consecutive regions. 

 This obviously may be the interpretation of the crossed 

 threads, but if as I suggested the opening out of the rings 

 themselves in different planes represents consistently a 

 reductional separation in a formerly twisted thread, then 

 the cross threads come to have a meaning, for they rep- 

 resent the level at which an earlier fusion and reunion of 

 the inner strands of the four strand stage took place. 

 From this point of view the cross strands, while having 

 nothing to do at this time with crossing over, neverthe- 

 less correspond to levels at which that process occurred. 



I do not wish to appear to be advocating the scheme 

 that I suggested as the best or as the only one that is in- 

 volved in crossing over. Any scheme that accounts by 

 means of twisting threads for interchange between the 

 segments of homologous chromosomes will fulfill suffi- 

 ciently the present requirements of crossing over. Much 

 more cytological and genetic work too will be necessary 

 before it is possible to state when and how this process 

 goes on. One point alone seems at present to be indi- 

 cated with some probability by the genetic evidence, 

 namely, that it would appear simpler for the interchange 

 to take place when the lines of genes are extended to the 

 fullest extent possible, and this would seem most easily 

 to take place, in the accurate way indicated by the genetic 

 facts, when the leptotene threads have spun out to their 

 farthest extent. Whether Janssens also ascribes to this 



