Fig. 507.—Formation of the blastoderm in Hydrophilus: b, completed blastoderm; d, yolk; f, so-called division-cells; k, “keimhautblastem”; z, yolk-cells.—After Heider, from Korschelt and Heider.
The embryo first arises as a whitish streak or band-like thickening on the ventral side of the egg, and is variously called the “primitive streak,” “primitive band,” “germinal band,” or “embryonal streak.” In most cases the primitive band is divided at regular intervals by transverse furrows, indicating the limits of what are to be the body segments.
Cross-sections (Fig. 509) show that the band is composed of several layers, i.e. an outer layer (ectoderm) and an inner layer which comprises the endoderm and mesoderm, and so long as these two layers are not sharply differentiated from one another, this second layer may be called, with Kowalevsky, “the inner lower layer, or ento-mesoderm” (Figs. 508, 509, B, C, u).
It is characteristic of insects, only rarely occurring in other arthropods (e.g. the scorpion), that the primitive streak is not situated on the surface of the egg, but becomes overgrown by a folded structure (Fig. 508, af) rising from its edges, the amnion-fold, so that it appears somewhat depressed or sunken in under the upper surface of the yolk. While the amnion-folds are extending from all sides over the primitive band, there becomes formed under it, by the invagination of the outer surface of the egg, a cavity, the amnion-cavity (ah), which, when the amnion-fold has completely overgrown the primitive band and united together (Fig. 509, C), appears completely closed from without.
Fig. 508.—Two schematic median sections through an insect-embryo to represent the development of the embryonal membranes. In A the primitive streak is not wholly overgrown by the amnion-fold. In B the amnion-folds have united with each other and completely overgrown the primitive streak: a, fore, b, hind, egg-pole; v, ventral side; d, dorsal side; af, amnion-folds; ah, amnion-cavity; am, amnion; do, yolk; ec, ectoderm; k, head-end, k′, hinder-end, of the primitive streak; s, the part of the serosa arising from the amnion-fold; s′, the part of the serosa arising from the unaltered blastoderm; u, lower layer.—After Korschelt and Heider.
Formation of the embryonic membranes.—The amnion-folds finally completely overgrow the primitive band (Fig. 509, B and C), and form the embryonal membranes. The primitive band is seen after its completion to be overgrown by a double cellular epithelial membrane. The outer of these two membranes, that which arises from the outer leaf or layer of the amnion-fold, is the serosa (Figs. 508, B; 509, C, s; 510). This passes continuously into the unchanged part of the blastoderm, which has no part in the formation of the primitive band and germ-layers, and which covers the outer surface of the yolk. Thus the serosa, which is usually held to include this portion also of the blastoderm, forms a closed sac which covers the whole surface of the egg, with one part extending over the surface of the yolk, and the other over the primitive band (Fig. 510).
Fig. 509.—Diagrammatic cross-section through three successive stages of the primitive streak, and growing embryonal membranes of insect-embryos. A, formation of the ventral plate and of the gastrula invagination (g). B, upward growth of the amnion-folds (af). C, complete overgrowth of the primitive band through the amnion-folds: v, ventral side; d, dorsal side; af, amnion-folds; ah, amnion-cavity; am, amnion; bl, blastoderm; bp, ventral plate; do, yolk; ec, ectoderm; s, serosa; u, under or inner layer.—After Korschelt and Heider.