THE PRINCIPLES OF OAK GALL FORMATION. 3 
definite on this point. ‘What is the first factor in 
gall formation ? This, the growth of the larva, as has 
already been pointed out. The second is that the egg 
must be laid in, or in contact with, the cambium layer ” 
(‘ Brit. Phyto. Hymen.,’ vol. iv, p. 21). “ Whena gall- 
fly has inserted its egg into the neighbourhood of this 
(cambium ring) tissue, what follows? In the first 
place the act of oviposition itself has no effect. 
IT am convinced that the simple prick of the gall-fly 
does not set up gall formation ; this, I hold, only begins 
when the larva emerges from the egg” (‘ Alternating 
Generations,’ p. 99). 
In comparison with the opinions of such specialists, 
supported by those of recent observers, and confirmed 
by many who have devoted years of research to the 
subject, the theories of gall origin advanced by very 
early writers have but little value. They must not, 
however, be entirely omitted. It was thought by 
several of the early observers that the production of 
cell activity was caused by a glandular secretion of an 
irritating nature, introduced with the egg at the time 
of oviposition. Others advanced a theory that the 
action of oviposition caused irritation to the plant. 
Another suggestion was made, viz. that the egg pos- 
sessed a higher temperature than that of the plant, and 
thus the cells were warmed into activity. Several 
writers have also maintained that infection, in the 
form of disease, was the sole cause of galls. 
That a secretion of some kind emanates from the 
gall-wasp when ovipositing is well known, and it can 
be seen, especially if a number of imagines develop from 
galls kept in a glass bottle or glass-topped box. They 
will place their eggs on the glass, and at the same time 
the minute drop of fluid can be seen also. Andricus 
nudus will provide a good example. It would appear 
that the purpose of the secretion is to seal the hole 
caused by the ovipositor. That it does not stimulate 
the plant to action the following will demonstrate. 
The imagines from galls of Trigonaspis crustalis make a 
