The arrangements and movements we have briefly alluded to have been limited to the unfertilised condition of the egg (we should rather say, the fertilising element has taken no part in them), and have as their result the union of the chromosomes existing after the extrusion of the two polar bodies, into a small body called the female pronucleus or egg-nucleus (Eikern), while the position of the movements has been an extremely minute portion of the egg near to its outer surface or periphery. The introduction of a sperm, or male, element to the egg through the micropyle gives rise to the formation of another minute body placed more in the interior of the egg, and called the sperm-nucleus. The egg-nucleus, travelling more into the interior of the egg, meets the sperm-nucleus; the two amalgamate, forming a nucleus or body that goes through a series of changes resulting in its division into two daughter-bodies. These two again divide, and by repetitions of such division a large number of nuclei are formed which become arranged in a continuous manner so as to form an envelope enclosing a considerable part (if not quite the whole) of the egg-mass. This envelope is called the blastoderm, and together with its contents will form the embryo. We must merely allude to the fact that it has been considered that some of the nuclei forming the blastoderm arise directly from the egg-mass by a process of amalgamation, and if this prove to be correct it may be admitted that some portions of the embryo are not entirely the result of division or segmentation of combined germ and sperm-nuclei. Wheeler states[[73]] that some of the nuclei formed by the first differentiation go to form the vitellophags scattered throughout the yolk. We should also remark that, according to Henking, the blastoderm when completed shows at one part a thickening, immediately under which (i.e. included in the area the blastoderm encloses) are the two polar bodies, which, as we have seen, were formed by the germinating body at an earlier stage of its activity. Fig. 79 represents a stage in the development of Pyrrhocoris, showing the interior of the egg after a body has been formed by the union of the sperm and egg-nuclei; this body is about to undergo division or segmentation, and the equatorial arrangement where this will take place is seen. The two polar bodies P1, P2, after having been excluded, are nearly reincluded in the egg.
The Ventral Plate.
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.
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 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.
We should observe that the terms ectoderm, mesoderm, and endoderm will probably be no longer applied to the layers of the embryo when embryologists shall have decided as to the nature of the derived layers, and shall have agreed as to names for them. According to the nomenclature of Graber[[75]] the blastoderm differentiates into Ectoblast and Endoblast; this latter undergoing a further differentiation into Coeloblast and Myoblast. This talented embryologist gives the following table of the relations of the embryonic layers and their nomenclature, the first term of each group being the one he proposed to use:—
Nussbaum considers[[76]] that "there are four layers in the cockroach-embryo, viz. (1) epiblast, from which the integument and nervous system are developed; (2) somatic layer of mesoblast, mainly converted into the muscles of the body-wall; (3) splanchnic layer of mesoblast, yielding the muscular coat of the alimentary canal; and (4) hypoblast, yielding the epithelium of the mesenteron."
