XX Introduction. 



The oak gall-flies about to be described will be found 

 to afford examples of heredity occurring among purely 

 agamous species, as well as among those which alternate 

 between an agamous and a sexual generation. The 

 characters of each generation are inherited and passed 

 on in two perfectly distinct streams. The advance 

 which has in recent times been made towards a clearer 

 comprehension of heredity, is in great measure due 

 to the influence of Weismann \ who, by discarding the 

 idea that sexual reproduction is in any way funda- 

 mental or essential to life, has led us to regard the 

 facts of heredity wholly untrammelled by it. Amphi- 

 mixis undoubtedly has its advantages, but descent may 

 be continuous in the female line, or there may even 

 be a male parthenogenesis ^ 



To understand the mechanism by which alternation 

 of generations is brought about, it is necessary to recall 

 the minute structure of the sexual cells, and especially 

 the behaviour of their nuclear contents ; and to observe 

 the difference in the extrusion of the polar cells, as 

 occurring in the eggs of parthenogenetic and sexual 

 species. 



The changes in the ovum during nuclear division 

 are briefly these ''. After a resting stage the chromatin 

 granules, which there is reason to believe are the 

 material bearers of hereditary qualities, appear as 

 a thread, apparently, spirally coiled within the nucleus. 

 An accessory nucleus forms and divides into two ; 

 at each pole of its achromatic spindle is placed 

 a centrosome with its surrounding attraction-sphere ; 

 the nuclear membrane disappears ; the chromatin 



^ Weismann, Essays on Heredity^ vol. ii. p. 86, 1893. 



^ Ibid. vol. i. p. 253; Schenck, Handbiich der Botanik, Bd. ii. p. 219. 



^ Weismann, Essays on Heredity, vol. ii. p. 118. 



