Galls by Gall- Flies. 107 



ferencewith embryonic development, but I must confess 

 that I have seldom found a dead egg which had not 

 reached development. The cause lies elsewhere. It 

 was constantly remarked that, in the different culture 

 experiments made, the greater proportion of the galls 

 failed. This was most easily observed in the case of 

 flies that produced bud-galls, and which, therefore, lay 

 only one egg in the bud. Naturally the species 

 emerging in summer can only prick winter buds which 

 are awaiting the next period of vegetative activity. In 

 this circumstance we may discover one reason for the 

 failure of the galls, for we may assume that, in many 

 seasons, a premature and anomalous development of 

 winter buds may be absent. But, according to all 

 observations, this cannot be the only reason ; indeed, 

 the first essential to insuring gall growth is the 

 right position of the egg. Gall formation will not fail 

 if the larva, when it emerges from the egg, meets with 

 its appropriate cell surroundings ; but to secure this 

 the fly must lay its egg with the greatest exactitude. 

 In the case of winter buds, it is especially important 

 that the egg be placed exactly in the zone of the 

 cambium-ring, which lies like a fine seam in the 

 base of the bud. We see that from the winter buds, 

 without exception, only bud-galls, and not leaf-galls, 

 are produced ; a proof that the larva has no power to 

 unfold the leaf-germs, but that gall formation can only 

 proceed from the zone of the cambium-ring ; and if 

 the egg should not have been laid by the fly exactly 

 where the emerging larva could reach this fine zone, 

 it perishes without forming a gall. If we think of the 



