The next important change after the formation of the blastoderm is the partial detachment of a part of its periphery to become placed in the interior of the other and larger portion. The way in which this takes place will be gathered from the accompanying diagrammatic figures taken from Graber: a thickened portion (_a_ _b_) of the blastoderm becomes indrawn so as to leave a fold (_c_ _d_) at each point of its withdrawal, and these folds afterwards grow and meet so as to enclose the thickened portion. The outer envelope, formed in part by the original blastoderm and in part by the new growth, is called the serosa (_e_ _f_), the inner layer (_g_) of the conjoined new folds being termed the amnion: the part withdrawn to the interior and covered by the serosa and amnion is called the ventral plate, or germinal band (_Keimstreif_), and becomes developed into the future animal. The details of the withdrawal of the ventral plate to the interior are very different in the various Insects that have been investigated. [Illustration: FIG. 80.—Stages of the enclosure of the ventral plate: A, _a_, _b_, ventral plate; B, _c_, _d_, folds of the blastoderm that form the commencement of the amnion and serosa; C, _e_, _f_, part of the serosa; _g_, amnion.] One of the earliest stages in the development is a differentiation of a portion of the ventral plate into layers from which the future parts of the organisation will be derived. This separation of endoderm from ectoderm takes place by a sort of invagination, analogous with that by which the ventral plate itself is formed. A longitudinal depression running along the middle of the ventral plate appears, and forms a groove or channel, which becomes obliterated as to its outer face by the meeting together of the two margins of the groove (except on the {149}anterior part, which remains open). The more internal layer of the periphery of this closed canal is the origin of the endoderm and its derivatives. Subsequently the ventral plate and its derivatives grow so as to form the ventral part and the internal organs of the Insect, the dorsal part being completed much later by growths that differ much in different Insects; Graber, who has specially investigated this matter, informing us[74] that an astonishing multifariousness is displayed. It would appear that the various modes of this development do not coincide with the divisions into Orders and Families adopted by any systematists.