SECT. 3] AND ORGANISATION 609 



of embryonic mortality, which will be mentioned again in Section 18, 

 and which provides a striking temporal field for the display 

 of genetic characteristics. To take only one example, Dunn & 

 Landauer studied the relative embryonic mortality of a variety of 

 chick called the "creeper". Cutler had first reported that, in 

 "creeper" fowls, the leg and wing bones were shorter and thicker 

 than in normal fowls, and that creepers never bred true, but usually 

 produced {a) normal chicks, {b) creeper chicks and (c) chicks with 

 extreme leg defects. The case was thus analogous to that of the yellow 

 mouse, which is always heterozygous, because the homozygous yellow 

 embryos die early in development. The relative embryonic mortalities 

 were found to be as follows : _^ 



Percentage of total embryos incubated. 



These results lead naturally to the assumption that the high early 

 mortality of the creeper x creeper matings was due to the death 

 of homozygous creeper embryos early in development. The creeper 

 variation would thus seem to be due to a single dominant gene which 

 is lethal in the homozygous condition. But the important point for 

 this discussion is that, if the point of action of a gene can be found 

 to occur at a definite point in development, a new outlook in genetics 

 becomes possible, for what we know to be taking place in the physio- 

 logical and chemical activity of the embryo at that period may tell 

 us a good deal about the gene itself. 



Much thought has been given to these questions in recent years. 

 Danforth, for instance, asked the question whether genes interact 

 with one another during embryonic development to produce struc- 

 tural and functional characters, or whether they each exert their 

 influence separately, thus 



Experiments with mice led him to regard the latter view as 

 the more probable. The three known effects of the Y-gene occur 



N EI 39 



