Sexual and Parthenogenetic Forms 335 
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 1815 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 earlier 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 was shown 
later, however, that the embryo arises from an egg produced in 
an ovary. The ovary and the way in which 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 little real basis for calling 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, with 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 winged individuals often occur. This 
1 The phylloxerans, for example. 
