OF EPPING FOREST. 26g 



forming the means of exit for some small parasite may often be 

 mistaken for that of the gall-maker. 



Parthenogenesis and Alternation of Generations. 



Alternation of generations is an exceedingly interesting 

 feature occurring in the life history of those Cynipidae forming 

 galls on the oak. There are two generations a year, the first, 

 consisting of females only, is known as the Agamic generation, 

 the second, which includes both males and females, as the Sexual 

 generation. 



The two generations give rise to galls which may differ 

 entirely both in shape, size and situation. 



Those galls which mature in the spring give rise to insects 

 comprising females only, which proceed to lay fertile eggs with- 

 out having any connection with a male. This is known as 

 Parthenogenesis or reproduction without male agency, the 

 generation of flies being Agamic. After these have oviposited 

 in the spring, galls arise which mature in the summer, and the 

 insects emerging from them comprise both males and females 

 (sexual generation) ; the latter, after copulation, lay eggs which 

 in turn eventually cause galls to form which are exactly similar 

 to those from which the agamic generation emerged in the 

 spring. The reason for this alternation of generations is not 

 known, and it would take too long to discuss the various theories 

 on the subject, but it seems to be intimately connected with the 

 extrusion of the polar bodies during the germination of the egg, 

 previous to the formation of the embryo. In the eggs of the 

 sexual generation two polar bodies are extended, while in the eggs 

 laid by the agamic generation only one polar body is extruded. 



In all lower forms of life we find the occurrence of partheno- 

 genesis m.ore common than in the higher forms. As regards the 

 Cynipidae there are a few species forming galls on the oak 

 which have only one generation a year, this generation being 

 agamic. It is probable that these and other agamic generations 

 at one time possessed males, as we find cases among the 

 Cynipidae forming galls on other plants where a few males are 

 still present. 



For instance, in the case of Rhodites rosae, which forms the 

 common " Bedeguar " or Pincushion-gall on the rose, the males 

 are almost extinct, though a few are still found ; consequently 



