T4 Observations on Cynipidae. 



observed the same thing happen when oaks growing in 

 the open air have been pricked, and I am therefore 

 compelled to attribute to meteorological conditions 

 a most important influence over the development of the 

 ^%%. The appearance of the flies almost always takes 

 place about the same time, and embryonic development 

 begins immediately after the eggs are laid. Absolute 

 rest in the evolution of the ^^^ never occurs, for even if 

 the temperature should be very low the formation of 

 the blastoderm begins at once. Naturally this proceeds 

 more slowly in a cold than in a warm season. I have 

 satisfied myself by various experiments that when 

 pricked buds are kept in a warm room the several 

 stages of embryonic development run their course 

 much more quickly than they do in buds kept in the 

 open air. In any case the embryo reaches its full 

 development in a few weeks. It may happen, however, 

 that at the particular time when this takes place vegeta- 

 tion may be backward, and the circulation of the sap 

 may not yet have begun in the tree, nevertheless the 

 time when the development of the embryo is completed 

 is just the time when gall formation should make 

 a beginning. As long as the embryonic envelopes 

 remain intact gall formation does not begin, but it starts 

 the moment the larva frees itself from the ^%% coverings. 

 Around the larva cell proliferation now commences and 

 this corresponds to the first beginning of gall formation. 

 But the production of this cell proliferation is con- 

 ditional, and depends on the state of vegetable growth ; 

 the sap, the pabulum of which the cells are formed, 

 must be in circulation. When cold weather retards 



