272 Experimental Zoology 
Beyerinck made a careful study of the galls of the willow 
(Salix amygdalina) produced by the gallfly, Nematus capree. 
This case is especially interesting, since the galls begin almost at 
once to develop, and may be full sized before the larva hatches. 
The cause of the growth in this case is the albuminous secretion 
that the gallfly injects along with the egg into the leaf. That 
the secretion and not the egg is the cause of growth has been 
shown by puncturing, and thus killing the egg with a fine 
needle. The gall continued to develop. This gallfly deposits 
its egg in the young leaves of the willow in the early spring. 
The gall can be detected within two days and has finished its 
growth in three weeks. The larva feeds on the inner wall of 
the gall, and finally bites a hole in one side through which it 
escapes after the gall falls to the ground. It then spins a cocoon 
and in August the adult fly, the second generation, emerges. 
It seeks young growing buds of the willow and pierces them. 
The gall develops in the autumn and falls to the ground with the 
leaves., The larva spins its cocoon, inside or outside the gall, 
and overwinters in this condition. In the first generation, in 
the spring, there are no males; in the second generation occa- 
sionally males may be found, but nevertheless Beyerinck thinks 
that this generation also reproduces by parthenogenesis. 
Beyerinck has discussed the question of the kind of changes 
that occur when a gall is produced, and he has carried out some 
experiments that bear on this important question. He tried 
to determine whether the cells of the gall are permanently 
changed, 7.e. whether their structure has been so affected that 
whatever they produce will be different from the tissues of the 
parent plant from which they arise; or whether the change is 
only temporary, depending on the presence of some substance 
exciting them to a peculiar mode of growth. The latter view 
seems to him the more probable as shown by the following facts: 
If the leaves are removed from a twig bearing the so-called 
“willow-rose’’ gall, new buds grow out of the axils of the leaves 
of which the gall is made. ‘The first leaves are somewhat modi- 
fied, like those of the “‘willow-rose,’’ but the later leaves are like 
