ii6 THE BIOLOGY OF INSECTS 



an egg-nucleus and a sperm-nucleus. This process is often 

 called the fertilisation of the egg, and fertilisation is usually 

 a necessary preliminary to reproduction. As the egg is 

 relatively large and passive and the sperm is very small and 

 active, the latter moves towards the former ; this it can do 

 because it possesses, besides the ovoid or rod-Hke head in 

 which the nucleus lies, a long vibratile tail or flagellum by 

 means of which it can swim through fluid. When pairing 

 (copulation) takes place between a male and a female insect 

 a large number of sperms are passed by the former into the 

 reproductive system of the latter. They are stored in a 

 special receptacle (spermatheca) so that they may subse- 

 quently fertiHse the eggs before these are laid. Among 

 insects, therefore, fertilisation may not occur until some 

 time after pairing. Fertilisation — the union of two gamete- 

 nuclei to form a zygote-nucleus — is the essential starting- 

 point for normal sexual reproduction. 



From this brief account it will be realised that the 

 germ-nuclei must play a very important part in the pro- 

 cesses of reproduction and inheritance, and these nuclei 

 are bodies of very small size. A male insect's sperm of 

 average dimensions is about 3^0 rnn^- (= ihi inch) in length, 

 and its head, enclosing the nucleus, may be no more than 

 one- tenth the length of the tail or flagellum. As the head, 

 with the adjacent '' centre-piece," is all that usually enters the 

 egg Sit fertihsation, it is clear that this body must contain all 

 the material necessary for the transmission to an individual 

 of the next generation of the characters, either of structure 

 or habit, that are inherited through the male parent. The 

 formation of the mature flagellate sperms results from a 

 process of development that may often be traced back to 

 an early stage in the Hfe of a male insect. Some essential 

 facts about this development (spermatogenesis) may now 

 be profitably considered as an introduction to our study of 

 inheritance. 



Sperm-formation is the result of a special type of cell- 

 division, and it is well known that the development, growth, 

 and maintenance of any living body are due to a series of 



