Fig. 537.—Two successive stages in the gastrulation of Apis. Cross-section through the primitive band: b, lower (inner) layer; ec, ectoderm.—After Grassi, from Korschelt and Heider.
During the invagination of the middle plate and its transformation into the gastrula-tube a change takes place in its histological character (Fig. 539, A and B). While it originally consists of a high cylinder epithelium, which after farther changes becomes divided into several layers, since the wedge-shaped single cells push themselves over each other, the cells in later stages become more and more cubical or irregularly polygonal (Fig. 539, B), and are irregularly arranged. At the same time the gastrula-tube is compressed in a dorso-ventral direction. While it in this way spreads out laterally under the side plates (ectoderm), its originally circular primitive lumen passes into the form of a horizontal fissure, which in Hydrophilus long remains as the boundary between the two layers of the inner (or lower) membrane. (Korschelt and Heider.)
There are numerous variations of the process of gastrulation, which are by Korschelt and Heider divided into three types, as follows:—
1. Through invagination and formation of a tube (Fig. 539, A, Hydrophilus, Musca, Pyrrhocoris, etc.).
2. By a lateral overgrowth (Fig. 537, Lepidoptera and Hymenoptera).
3. By an inward growth of cells from a median furrow (Aphides and Trichoptera).
In Doryphora and Lina (Fig. 524) the hinder end of the gastrula furrow is forked.
Fig. 538.—Diagrammatic sketch of the formation of the germinal layers in Doryphora: A, view of upper surface. B, cross-section through the fore end of the primitive streak at the line a-a. C, section through the middle of the primitive streak corresponding to the line b-b. D, section through the hinder end of the primitive band corresponding to the line c-c: bl, blastopore; ec, ectoderm; en′, anterior U-shaped; en″, hinder U-shaped germ of the endoderm; ms, mesoderm.—After Wheeler, from Korschelt and Heider.
The cellular layer arising from the gastrula invagination (lower layer) forms the common germ of the endoderm and mesoderm. It has only recently become known how these two germ-layers of insects have become differentiated. Kowalevsky first discovered in Musca that the greatest part of the lower (inner) layer yielded mesoderm exclusively, and that a cell-mass only corresponding to the most anterior and posterior end of the primitive band was used in the formation of the endoderm. We must therefore, in insects, speak of a fore and a hinder endodermal rudiment. In proportion, now, as the ectodermal invaginations, which are destined to form the stomodæum and the proctodæum sink beneath the surface of the embryo, the cell-masses of which the two endodermal rudiments are composed are pushed farther in, and a separation between them and the mesoderm is thus effected. The two endodermal rudiments now form accumulations of cells which lie closely adjacent to the blind ends of the stomodeal and the proctodeal invaginations. They soon widen out into two hour-glass-shaped rudiments, which are directed with their concavities towards each other, but with their convex side towards the nearest pole of the egg. They soon change their form; two lateral stripes grow out from them, and each now assumes the form of a U (Fig. 538, en′). The limbs of the fore and hind U-shaped rudiment are directed toward each other, and grow towards each other until they meet, and are fused together. Thus the endodermal rudiments arising out of the fusion of the two U-shaped rudiments form two stripes extending along the primitive band and situated mostly under the primitive segments. At the two ends the endodermal rudiment fuses with the stomodeal and proctodeal invaginations. These lateral endodermal streaks now spread out, and gradually begin to grow over the yolk, on whose outer surface they lie. This overgrowth makes the greater advance on the ventral side, so that the two endodermal streaks first unite in the ventral median line and afterward in the dorsal. The yolk in this way passes completely into the interior of the rudiment of the mid-intestine.