Sexual and Parthenogenetic Forms 323 



The first experiment showing that external influences have 

 an effect on the mode of reproduction was that of Dageer in 

 1773, who kept plants containing aphids in a warm place, 

 and found that the sexual forms did not appear. The cycle of 

 the species with which he worked was shown, therefore, to be 

 an open one, its completion depending on external conditions. 

 Kyber in 181 5 raised fifty successive generations of partheno- 

 genetic individuals during four years by keeping the animals 

 and their food plants in the warmth during the winter. It 

 would be interesting to know whether, in warm climates where 

 the plants — the rose, for instance — grow throughout the year, 

 the sexual form ever appears. 



The earher students of parthenogenetic development of the 

 aphids were much puzzled over the viviparous production of 

 young in the absence of males, and it was thought by some ob- 

 servers that the development could not be by means of eggs, but 

 was the result of a process of internal budding. It w^as show^n 

 later, however, that the embryo arises from an egg produced in 

 an ovary. The ovary and the way in w^hich the egg appears in 

 it are nearly the same as in other related species in which eggs 

 are deposited ; but a remarkable condition exists in the vivipa- 

 rous aphids : the egg begins to develop almost immediately after 

 it has left the ovary, when it is very small and when from com- 

 parison with other species it appears to be in a very immature 

 condition. However, since the ripening process of the nucleus 

 is the same as in other parthenogenetic eggs that are larger, and 

 are regularly deposited,^ there is Httle real basis for calHng the 

 aphid egg immature, because it is small when it begins to de- 

 velop. The commoner forms of aphids, the rose aphid, for 

 example, pass through the following series of forms, wdth some 

 variations. In the spring, the eggs (that had been deposited on 

 the stems of the food plant) complete their development, and 

 parthenogenetic wingless females emerge. These grow^ rapidly 

 to the full size, and produce a new generation of wingless forms 

 amongst which a few w^inged individuals often occur. This 



^ The phylloxerans, for example. 



