202 ROSCOE R. HYDE 



The hybrid A is made up of T 9 and W cf . Let us assume that 

 its germ cells segregate into the parental types in so far as factors 

 for fertility are concerned. Accordingly A 9 produces two kinds 

 of eggs, T eggs and W eggs. A cf will produce two kinds of sperm, 

 T sperm and W sperm. Diagram H expresses the relation as to 

 what happens when the gametes are brought together. 



^9?' T 71) 



jSperm f 



Diagram H Showing the possible combinations of the gametes of the 

 hybrids (A) on the assumption that the germ cells have segregated into the 

 parental types in respect to factors for fertility. 



When a T sperm fertilizes a T egg 24 per cent of the eggs hatch 

 When a T sperm fertilizes a W egg 80 per cent of the eggs hatch 

 When a W sperm fertilizes a T egg 55 per cent of the eggs hatch 

 When a W sperm fertilizes a W egg 75 per cent of the eggs hatch 



The sum, 234, divided by 4 gives on this assumption the actual 

 fertilizing power of the combination, or 58.5, which corresponds 

 very closel}^ with the 56.6 observed. In like manner the other 

 combinations may be determined. I have placed in a circle in 

 each case the calculated percentage of fertility; the number out- 

 side the circle expresses the observed percentage. The agree- 

 ment between the two sets of numbers lends support to the view 

 that the germ cells of the hybrid do segregate into factors identical 

 with those of the parents in so far as factors that are responsible 

 for fertility are concerned. 



We find here, I believe, the key for the low fertility of the 

 truncates. They have become homozygous for many factors. 

 The corresponding rise in fertility when crossed into other races 

 is to be explained on the assumption that the combination formed 

 is heterozygous for more characters upon which fertility depends 

 and consequently more likely to develop. The assumption seems 

 a reasonable one to make in light of the fact that we are able to 

 predict the fertilizing power of the gametes in a given combination. 



