Development is the general term applied to all those changes which a germ undergoes before it assumes the characters of beyond reasonable doubt that the males are produced by a process of parthenogenesis. Landois, however, asserts that the eggs of insects are of no sex, that sex is only developed in the larva after its emergence from the egg, and that in each individual larva the sex is determined wholly by the nature of the food upon which it is brought up ; abundant nourishment producing females, and scanty diet giving rise to males.
The perfect individual; and the chief differences which are observed in the process as it occurs in different animals consist simply in the extent to which these changes are external and visible, or are more or less completely concealed from view. For these differences the terms "transformation" and "metamorphosis" are employed; but they must be regarded as essentially nothing more than variations of development.
Transformation is the term employed by Quatrefages to designate "the series of changes which every germ undergoes in reaching the embryonic condition; those which we observe in every creature still within the egg; those, finally, which the species born in an imperfectly developed state present in the course of their external life."
Metamorphosis is defined by the same author as including the alterations which are "undergone after exclusion from the egg, and which alter extensively the general form and mode of life of the individual."
Though by no means faultless, these terms are sufficiently convenient, if it be remembered that they are merely modifications of development, and express differences of degree and not of kind. An insect, such as a butterfly, is the best illustration of what is meant by these terms. All the changes which are undergone by a butterfly in passing from the fecundated ovum to the condition of an imago, or perfect insect, constitute its development. The egg which is laid by a butterfly undergoes a series of changes which eventuate in its giving birth to a caterpillar, these preliminary changes constituting its transformation. The caterpillar grows rapidly, and after several changes of skin becomes quiescent, when it is known as a "chrysalis." It remains for a longer or shorter time in this quiescent and apparently dead condition, during which period developmental changes are going on rapidly in its interior. Finally, the chrysalis ruptures, and there escapes from it the perfect winged insect. To these changes the term metamorphosis is rightly applied. These changes, however, do not differ in kind from the changes undergone by a Mammal; the difference being that in the case of a Mammal the ovum is retained within the body of the parent, where it undergoes the necessary developmental changes, so that at birth it has little to do but grow, in order to be converted into the adult animal.
From these considerations we arrive at a second generalisation, which is thus formulated by Quatrefages: "Those creatures whose ova - owing to an insufficient supply of nutritious contents, and an incapacity on the part of the mother to provide for their complete development within her own substance - are rapidly hatched, give birth to imperfect offspring, which, in proceeding to their definitive characters, undergo several alterations in structure and form, known as metamorphoses."