FROM EGG TO INSECT SNODGRASS 375 



therefore, the individuals of each species are endowed with the power 

 of reproducing: themselves. Some of the lower animals can regen- 

 erate themselves from pieces of their own tissues, but most animals 

 pi-ocreate b}'^ means of special cells that have the property, under 

 certain conditions, of reproducing a complete form like the parent. 

 These special reproductive cells are known as the germ cells. Some 

 of the germ cells of plants, called spores, have no distinction of sex, 

 but in animals the germ cells are always of two kinds — male and 

 female. The two kinds may be borne by the same individual, but 

 ordinarily each is carried by a different kind of individual of the 

 same species, and these individuals are accordingly distinguished 

 also as male and female. The visible differences between male and 

 female individuals, however, are mostly of a secondary nature, hav- 

 ing a relation to the different demands of the germ cells on the indi- 

 viduals that mature them; the fundamental differences of sex are in 

 the structure of the body cells themselves, which are distinguished 

 by the same characters as are the germ cells. 



The mature male germ cells are known as the spermatozoa^i or 

 sperms; the mature female germ cells as the ova, or eggs; but at 

 any stage these cells may be called the male and female germ cells. 

 Ordinarily the germ cells must be united in pairs of opposite kinds 

 in order to produce new individuals. This kind of development con- 

 stitutes sexual reproduction, and the union of the two cells is knowm 

 as fertilization. In some invertebrate animals, however, including 

 many species of insects, the Q^g is capable of developing alone, and 

 development of this sort is distinguished as asexual reproduction, or 

 parthenogenesis. The male germ cell never develops by itself; it 

 must always be united Avith at least a part of an egg cell. 



The female germ cell, then, being the one on which the responsi- 

 bility of development depends, is usually protected in some way from 

 destroying influences, either by being inclosed in a tough shell, or by 

 being retained wuthin the bodj^ of the mother during the earl}^ part 

 of its developmental period. If the egg is extruded from the body 

 of the parent and must develop after that without receiving any 

 nourishment from the mother, there is provided within its shell an 

 amount of nutrient material sufficient to carry it through to the time 

 when the newly formed creature can leave the shell and obtain food 

 for itself. This nutrient material within the egg is called yolh. 



The eggs of insects have a great variety of shapes, determined by 

 the shell (fig. 1). Some are round, some are oval, some are flat; 

 some are smooth on the outside, some ridged or variously sculptured. 

 Some are laid by the female in the most exposed places, others are 

 deposited where they will be protected, as under bark, in the ground, 



