Biorhiza Group. 145 



sooner ; nay, on the contrary, in September the period 

 of vegetation is on the wane. The most probable 

 explanation seems to be that this peculiarity of embry- 

 onic evolution has been inherited from the generation 

 Biorhiza aptera, in which the embryonic stage lasts fully 

 four months. 



Yet it is necessary to add that this explanation does 

 not hold good for all species ; thus, for example, Dryo- 

 phanta divisa has an embryonic stage even more ex- 

 tended, for the eggs are laid in October, and the gall 

 does not develop ' until May. In the corresponding 

 summer generation [Spathegaster verrucosus) the galls 

 are formed at once, and appear four weeks after the eggs 

 are laid. 



The eggs of the two generations, which are necessarily 

 always unfertilized in the one, and fertilized in the other, 

 do not present any longer period of rest in the one case 

 than in the other, but on the contrary all show signs of 

 commencing embryonic development immediately after 

 they have been laid. Parthenogenetic eggs, which are 

 almost always laid in the winter, simply exhibit a much 

 slower evolution than fertilized eggs, which are always 

 laid in the summer. Winter eggs thus offer very good 

 subjects for observation, on account of the slow progress 

 of the various steps of development ; it is however 

 a tedious and difficult process to extract them from the 

 buds uninjured, and to make good preparations of them. 

 As I have already said, all my attempts to overcome this 

 difficulty, by endeavouring to secure the evolution of 

 eggs taken from the ovary direct, have been fruitless. 

 However simple may appear the circumstances of the 



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