FERTILITY AND STERILITY IN DROSOPHILA 185 



given in table 12 came from the hybrids recorded in table 9. 

 The pairs given in table 13a, b came from the hybrids recorded in 

 table 11. 



Tables 12 and 13 appear to show that low production reappears 

 after skipping a generation and that this low production is trans- 

 mitted through both the egg and sperm to the grandchildren, 

 I shall not attempt to analyze the data further as I have other 

 and more exact evidence that bears on the transmission of the 

 low fertility of the truncates. 



CROSSES BETWEEN THE LOW-PRODUCING TRUNCATES AND THE 



HIGH-PRODUCING WILD STOCKS IN WHICH AN EXACT 



MEASURE OF FERTILITY IS EMPLOYED 



The term ' fertility ' is used in so many different ways by differ- 

 ent writers that I wish to make clear at the outset the sense in 

 which the term is used throughout the ensuing papers. By 

 'fertility,' I mean the percentage of eggs that complete develop- 

 ment and give rise to mature flies. For example, if 100 eggs are 

 isolated from a stock and later 50 flies come from these eggs then 

 I speak of the fertility of that stock as 50 per cent. It is evident 

 that fertihty in this sense can only be determined by isolating the 

 eggs, and that the number of offspring produced by a pair does not 

 give a measure of the fertility of that pair but only a measure of 

 the 'productivity' of that pair. 



The marked difference in the length of life between the truncate 

 stock and the wild strains together with the increased length of 

 life of the hybrid makes it very difficult to draw safe conclusions 

 in regard to the role played by the egg and sperm in fertilization 

 in so far as estabhshing any definite ratios are concerned. In 

 other words the production of offspring cannot be taken as an 

 absolute measure of the fertilizing power of the egg and sperm for 

 as we shall see later in the case of the hybrids, although their 

 production of offspring is very high, their fertility (combination 

 of egg and sperm) is relatively low. 



Accordingly, I wish to return to the question of the low fertility 

 of the truncates and consider the behavior of the low fertility 

 in this race on crossing into other races. In order to insure an 



