THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 281 



Aa mother are at first all alike, with dark ocelli. In later stages, the 

 ocelli become lighter in half the larvae and in the last larval instar the 

 two classes aa and Aa can be easily distinguished. One must assume 

 that in the second cross the aa animals at first develop pigment because 

 some precursor has been carried over into the egg from the Aa mother, 

 but that further pigment cannot be produced when once this store is 

 used up. Kiihn put this interpretation beyond doubt by showing that if 

 an A testis is implanted into an aa female, it releases a substance into 

 the blood which causes the darkening of the host's eyes ; and further, 

 the eggs laid by this aa female now possess an initial store of pigment 

 exactly as do the eggs of an unoperated Aa female. Maternal effects 

 are also known in silkworms,^ Gammarus,- and Drosophila,^ for 

 example. 



^ Toyama 1909. 



^ Sexton and Pantin 1927. ^ Dobzhansky and Sturtevant 1935. 



