194 ROSCOE R. HYDE 



Truncate >^Truncaie Inbred FJ, . Inbud r>4 



* i'Tto.eqqs isolated 



h'TIOiqqs hatched 



Diagram E Showing the effect on fertility of crossing the inbred and 

 truncate stocks. 



and if so this could be put to a test and at the same time more Hght 

 might be thro^vn upon the behavior of the truncate stock. Accordingly 

 the inbred stock which was now in the F14 generation was crossed into 

 the truncate stock. Table 16 gives the details of the experiment. 

 Table 17 is the control on the inbred stock that was bred in pairs at the 

 same time. 



The results of Experiment VI are expressed in diagram E. 



BACK CROSSES BETWEEN THE HYBRIDS AND THE RECESSIVE 

 LOW-PRODUCING TRUNCATES 



We may next consider the evidence that bears on the fertility 

 of the hybrids when paired with each other and the fertility of the 

 hybrids when back crossed into the recessive low-producing 

 truncates. This experiment was originally undertaken in order 

 to determine whether or not there were sex-linked factors for 

 fertility. The results in this direction have been largely negative 

 but nevertheless the experiments give us the key by which the 

 low productivity of the truncates may be explained on a very 

 simple assumption. 



It was my expectation to find that the eggs of the hybrids when 

 isolated would show a fertility of 100 per cent, since I had already 

 found that their production is far in excess of their high-producing 

 parent. Three experiments have been required in order to con- 

 vince myself that the fertility of the hybrids is in reality low and 

 that the percentage of fertility in the back crosses is an actual 

 measure of the fertilizing power of the egg and sperm. 



There are two classes of hybrids, as follows : Offspring from the 

 truncate 9 by Woods Hole cf will be referred to as A 9 and A cf . 



