102 On the Formation of 



the gall usually proceeds proportionately to the growth 

 of the cellular layer in which the egg is laid. Let us 

 take the simplest case, that of an egg laid in a leaf. 

 The gall formation begins in the la3^er of formative 

 cells on the under surface of the leaf, the layers of the 

 upper surface of the leaf are formed of cells which have 

 become stable ; they undergo no further change, nor 

 do they respond to any irritation applied to them. 

 They are incapable of forming new cells ; any new 

 formation can only originate in the cells of the under 

 surface. At first the gall formation only affects the 

 small zone of cells that surrounds it, but as soon as it 

 acquires a new vascular system of its own it begins to 

 grow apace as an independent structure. 



Matters are different when the eggs are laid in the 

 bud, and the emerging larva comes in contact with one 

 of the rudimentary leaves. These consist still of 

 unmodified cells, which are all equally capable of 

 development, whether they occur on the upper or under 

 surface of the leaf; both surfaces consequently take 

 part in gall formation, and when the leaf comes to 

 be unfolded it is found that therfe is an actual absence 

 of leaf tissue, and that the resulting gall grows through 

 the leaf substance. 



It is different again when the egg is laid in the 

 cambium-ring of the bark. Here the cells which first 

 form round the larva cannot be distinguished from the 

 adjacent cells of the cambium tissue, but in the later 

 cell-growth there is a sharp zonal contrast. The 

 peripheral zone of the cambium-ring produces the cells 

 of the bast parenchyma, while the central zone of the 



