REPRODUCTION AND HEREDITY 267 



insignificant normal size. There is no doubt that we observe 

 here a process of nutrition in which the epithelial cells only act, 

 as it were, as intermediaries which receive food-material from 

 the maternal body, store it within themselves, and transmit it, 

 as required, to the advancing egg-cells for their support during 

 development. When the mature eggs have left the ovary the 

 epithelial cells once more suck themselves full with the nutrient 

 fluid, fill once more the whole lumen, and once more give up 

 their contents to the next advancing group of eggs. It is sig- 

 nificant that this feeding process is only observed during the 

 production of the ' summer eggs,' but is entirely at rest when 

 the ' winter eggs ' are formed. This makes it also clear why 

 ' winter eggs ' require such a large number of egg-groups for their 

 development, and why their formation is only undertaken in 

 case of necessity. 



Such succession of sexual and parthenogenetic generations 

 is described as heterogony, to distinguish it from the typical 

 alternation of generation, the metagenesis. In numerous cases 

 heterogony is connected with a very prominent polymorphism ; 

 indeed, the members of the various generations are often so 

 divergently constructed that only after closely observing their 

 cycle of development it becomes possible to recognize them as 

 belonging to one species. 



Everyone has seen on oak-leaves the pretty red-cheeked gall- 

 apples, but less known are the little gall-flies whose sting produces 

 these growths. These gall-flies form a very numerous family 

 (Cynipida), those that live on the oak numbering alone about 

 a hundred species, each of which produces a different gall. 

 From the root to the fruit there is hardly a part which is not 

 exposed to the stings of the flies and cannot be compelled to 

 form such diseased growths. Though the gall-flies have a 

 special predilection for oaks, they do not on that ground despise 

 other plants, but attack the maple, fig-tree, wild rose, blackberry, 

 and many herbs. The bedeguar of wild roses, which is caused by 

 the sting of Ehodites rosce, and often assumes the size of 

 a fist, is known everywhere. But above all the Aleppo-galls of 

 the gall-flies of Asia Minor have attained a certain celebrity, 

 because they were formerly much employed in the prepar- 

 ation of very good ink. If we cut an oak-apple with a knife 



