The living matter contained in the egg of an Insect is protected by three external coats: (1) a delicate interior oolemm; (2) a stronger, usually shell-like, covering called the chorion; (3) a layer of material added to the exterior of the egg from glands, at or near the time when it is deposited, and of very various character, sometimes forming a coat on each egg and sometimes a common covering or capsule for a number of eggs. The egg-shell proper, or chorion, is frequently covered in whole or part with a complex minute sculpture, of a symmetrical character, and in some cases this is very highly developed, forming an ornamentation of much delicacy; hence some Insects' eggs are objects of admirable appearance, though the microscope is of course necessary to reveal their charms. One of the families of butterflies, the Lycaenidae, is remarkable for the complex forms displayed by the ornamentation of the chorion (see Fig. 78, B). [Illustration: FIG. 77.—Upper or micropylar aspect of egg of _Vanessa cardui_. (After Scudder.)] The egg-shell at one pole of the egg is perforated by one or more minute orifices for the admission to the interior of the spermatozoon, and it is the rule that the shell hereabouts is symmetrically sculptured (see Fig. 77), even when it is {145}unornamented elsewhere: the apertures in question are called micropyles. They are sometimes protected by a micropyle apparatus, consisting of raised processes, or porches: these are developed to an extraordinary extent in some eggs, especially in those of Hemiptera-Heteroptera (see Fig. 78, C). Some of these peculiar structures have been described and figured by Leuckart.[70] The purpose they serve is quite obscure. [Illustration: FIG. 78.—Eggs of Insects: A, blowfly (after Henking); B, butterfly, _Thecla_ (after Scudder); C, Hemipteron (Reduviid).] FORMATION OF EMBRYO. The mature, but unfertilised, egg is filled with matter that should ultimately become the future individual, and in the process of attaining this end is the seat of a most remarkable series of changes, which in some Insects are passed through with extreme rapidity. The egg-contents consist of a comparatively structureless matrix of a protoplasmic nature and of yolk, both of which are distributed throughout the egg in an approximately even manner. The yolk, however, is by no means of a simple nature, but consists, even in a single egg, of two or three kinds of spherular or granular constituents; and these vary much in their appearance and arrangement in the early stages of the development of an egg, the yolk of the same egg being either of a homogeneously granular nature, or consisting of granules and larger masses, as well as of particles of fatty matter; these latter when seen through the microscope looking sometimes like shining, nearly colourless, globules.