REPRODUCTION AND HEREDITY 115 



of a race. These problems have claimed much attention 

 from naturalists in recent years, and the partial solution of 

 some of them has been reached through studies on insects 

 by various methods of research, such as the careful micro- 

 scopic examination of their germ-cells and the tracing out 

 of factors in their inheritance by means of experimental 

 breeding. 



Among living beings there are two well-known methods 

 of reproduction, firstly by means of small living units 

 known as germ-cells, secondly by the strong outgrowth of a 

 portion of the organism which may be termed a bud, the 

 bud often separating subsequently from the parent-body. 

 Any kind of reproduction that can be regarded as budding 

 is very rare among insects ; they '' increase and multiply " 

 as a rule directly from the germ-cells, to which there- 

 fore attention may now be directed. 



It has already been mentioned in our Introduction 

 (Chap. I, p. 10) that the germ-cells of insects, as of animals 

 generally, are of tv/o kinds : minute sperm-cells, or sperma- 

 tozoa, active and mobile ; and comparatively large egg- cells 

 or ova, passive in behaviour and containing more or less 

 food material or yolk (Fig. 3). The sperms are found in the 

 sets of animals that we call males and the eggs in females ; 

 hence the two kinds of germ-cell are sometimes termed 

 respectively male and female cells, but this mode of expres- 

 sion may lead to misunderstanding. It is convenient to 

 have in use a term which can be applied either to a sperm 

 or to an egg, as many features of essential importance in 

 reproduction and inheritance are common to both ; such 

 a term is provided in the word gamete. Many animals — 

 earthworms and snails, for example — have both kinds of 

 gamete developed in the same individual, which is therefore 

 neither exclusively male nor female but hermaphrodite. 

 Among insects, however, hermaphrodites are extremely 

 uncommon. 



Reproduction by means of gametes is known as sexual, 

 and for normal sexual reproduction there must be union 

 between the nuclei of gametes of either kind, that is between 



