196 THE SECOND-CHROMOSOME GROUP 



i 



from the relation previously observed in the age- variations, and would 

 seem to indicate that the age and temperature variations were accom- 

 plished by different mechanisms by effects upon different physio- 

 logical factors. Those double cross-overs that do occur have the same 

 distribution along the chromosome at all temperatures, which shows 

 that the method of handling the chromosomes is unchanged. In 

 accordance with the analysis already given (p. 188), the cause of the 

 temperature variations in crossing-over is to be sought rather in vari- 

 ation in the coefficient of crossing-over in the crossing-over capacity 

 of the chromosome itself, because of some variations in its structure or 

 framework. Just as in the relation between the coincidence and 

 temperature curves discussed before, the coincidence curve calcu- 

 lated for such a tube-count temperature-time curve apparently cuts 

 through the rise and fall due to temperature instead of being altered 

 concomitantly with it. 



The conclusion just drawn from the failure of the temperature- 

 variation to affect the coincidence, namely, that the change in the 

 amount of crossing-over is probably due to a change in the physical 

 properties of the chromosome subsfcance, has an important bearing 

 on the question of the stage at which crossing-over itself occurs, as 

 follows: From a study of the time taken for the effects of exposure to 

 abnormal temperature to become manifest or to disappear, Plough 

 concluded that the effect was produced at one stage only in the devel- 

 opment of the ovary, and that eggs which have not arrived at or have 

 passed this critical stage are incapable of registering any temperature- 

 variation. It was next argued that this critical stage is that at which 

 crossing-over itself normally occurs. It seems certain that crossing- 

 over does not take place before this critical stage is reached, but it 

 does not follow that it might not occur at some stage between this and 

 the maturation divisions, that is, at any later stage during the growth 

 period. At the critical stage one of the factors which modifies the 

 frequency of the crossing-over becomes fixed, but the crossing-over 

 itself may occur later. As a crude analogy, the process of crossing-over 

 might be likened to a machine, say a saw-mill. The rate at which 

 boards are sawn depends, other factors remaining constant, upon 

 the toughness of the log fed against the saws, which toughness is a 

 physical property of the log fixed long previously. 



The coincidence analysis indicates that the setting of the crossing- 

 over machine has not been altered, but that the chromosome at a 

 specific sensitive stage in its fabrication has been modified in one of 

 its properties its toughness, let us say so that when it ultimately 

 undergoes crossing-over the output is different. 



It is quite possible that the crossing-over follows immediately after 

 the determination of this property; indeed, from other lines of evidence 



