CROSSING OVER AND CHROMOSOMES 113 



over — for which there is no evidence — one member of the 

 pair must lie on one side of its mate in one region, and 

 on the other side in other regions. Second, when the thick 

 thread splits anew just before condensing into the tetrad 

 it is so difficult to follow the course of the split in all cases 

 that it cannot be affirmed that it always lies in one plane 

 throughout the length of the chromosome, but if such 

 should turn out to be the case, as so often figured, it would 

 appear to mean that the crossing over had taken place and 

 been obliterated by the time the condensation began. 

 Third, evidence such as that described by Wenrich — of 

 which sort there are other cases but none quite so clear — 

 indicates that the chromosomes are enclosed in vesicles 

 until they begin to spin out each into a long thread. Inter- 

 change of the sort called for by the genetic evidence could 

 scarcely take place until the walls of the sacs had disap- 

 peared. The thin thread stage that follows would seem 

 best to fulfill the conditions called for by the genetic evi- 

 dence. The moment the primary split appears after the 

 two threads have fused there would seem to be precluded 

 any further chance for crossing over, as the genetic 

 evidence suggests. This analysis leads, then, to the thin- 

 thread stage as the most favorable stage for the requir- 

 ments of the genetic evidence. 



It is well known that most of our information about the 

 maturation stages is derived from the male, because of the 

 greater ease of obtaining the critical stages, and in prepar- 

 ing material. We are handicapped in discussing crossing 

 over to a large extent by the fact that we must appeal 

 largely to the evidence of spermatogenesis. In Drosophila 

 at least there is no crossing over in the male. On the 

 other hand, Nabours has recently found evidence in one of 

 the grasshoppers that crossing over occurs both in the 

 male and female. In this case evidence from the male 

 would be more to the point. Whether genetic crossing 

 over occurs in the male of Batracoseps and To^nopteris, 

 we do not know. 

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