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 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

^ The phylloxerans, for example.