270 THE THEORY OF THE GENE 



and females that appear at the end of a long succession 

 of generations of maggots reproducing by partheno- 

 genesis. 



The eggs laid by the winged females are supposed to 

 be fertilized by sperm from the winged males and develop 

 as far as the maggot (larval) stage. These maggots, with- 

 out passing on to the adult stage, produce eggs that de- 

 velop by parthenogenesis. From these eggs a new genera- 

 tion of maggots arises that repeats the process. This 

 continues throughout the year, the maggots living under 

 the bark of dead trees, and in some species on mushrooms. 

 In the spring or summer, winged males and females 

 appear from eggs laid by the last generation of maggots. 

 The appearance of the winged forms seems to be con- 

 nected with some change in the environment. Recently 

 Harris has shown that when the cultures become crowded, 

 owing to the presence of many maggots, the adult insects 

 appear if suitable conditions prevail, while if the mag- 

 gots are reared in isolation, or in small numbers, they 

 continue to reproduce in the larval stages (paedogene- 

 sis). The effective factors in crowding are not known. 

 If young from a single individual maggot are reared 

 together, and if their offspring in turn are kept in the 

 same culture, etc., it has been found by Harris that when 

 the adult flies appear they are of one sex in each such 

 culture. This seems to mean that each individual maggot 

 is either male or female in genetic constitution, and re- 

 produces by parthenogenesis the same sex. If this is the 

 correct conclusion, it follows that both the male-deter- 

 mined maggots and the female-determined maggots pro- 

 duce functional eggs. As yet we have no evidence relating 

 to the distribution of the sex-chromosomes in these flies. 



There is, here, an example of male-determined indi- 

 viduals producing parthenogenetic eggs at one phase of 

 the life cycle and spermatozoa at another phase. 



