REPRODUCTION 165 



cells. It is not uncommon, however, to find species of invertebrates 

 among which, for considerable periods of time, no males can be found. 

 The females produce eggs which develop into new individuals like the 

 parent, although fertilization by spermatozoa does not occur, since no 

 males are present. By their origin and division and nuclear changes 

 the cells giving rise to new individuals are ova; hence the method is 

 regarded as a sexual one. Development of an egg without fertilization 

 is known as parthenogenesis. There are many animals which employ 

 parthenogenesis. Some insects which do so are the plant lice, or aphids, 

 and many ants, bees, and wasps. The method has also been observed 

 in a few moths, a few of the scale insects, and commonly among the 

 flower-inhabiting insects known as thrips. 



The females of many parthenogenetic species produce, for a number 

 of generations, only females. At intervals, frequently in the fall, males 

 are also produced which fertilize the eggs. These zygotes usually differ 

 from the unfertilized eggs in being provided with hard shells and in being 

 resistant to the rigors of a winter season. The fertilized eggs hatch in 

 the spring into parthenogenetic females which repeat the cycle as out- 

 lined. Many species of aphids and of the lower Crustacea have cycles 

 of this type. In certain insects the bisexual reproductive phase is 

 apparently entirely omitted, and reproduction is exclusively partheno- 

 genetic. Thus the black flower thrips Anthothrips niger, the brown 

 chrysanthemum aphid Macrosiphum sanhorni, many species of scale 

 insects, and some gall-producing and parasitic insects never produce 

 males. In the ants, bees, and wasps, both males and females are usually 

 produced. The female lays both fertilized and unfertilized eggs, in some 

 way controlling fertilization of the eggs by the release or retention of 

 spermatozoa stored in the seminal receptacles. Among bees the males 

 (drones) are derived from unfertilized eggs, the females (queens and 

 workers) from fertilized eggs. 



Fertilization, where it occurs, has a dual function, that of (1) stimu- 

 lating the egg to develop, and (2) introducing the hereditary properties 

 of the male parent. In parthenogenesis there is only one parent; hence 

 no paternal qualities can be transmitted, and the eggs are able for 

 some reason to start development without any stimulus from a 

 spermatozoon. 



Parthenogenetic development has been induced in the eggs of a 

 number of animals which ordinarily require fertilization. The methods 

 have been various. Bathing the eggs with weak solutions of chemical 

 substances, shaking them vigorously in a bottle, heating them, or pricking 

 them with a fine needle, all have started division in certain eggs. Most 

 of the individual animals whose development was started in this arti- 

 ficial way have died in early stages, but a few frog eggs pricked with a 



