The gastrula stage.—The primitive band invaginates so as to give the opportunity for the formation of the inner layer. This invagination, which at a certain stage is established along the whole length of the primitive band, forms a median furrow and may be regarded as the gastrula-invagination of insects. The lower (inner) layer thus arising afterwards spreads out under the entire primitive band (Fig. 509, B and C), the edges of which become bordered by the growing amnion-fold. (Korschelt and Heider.)
In certain forms the primitive band arises from several separate rudiments which afterwards unite. Thus in Musca and Hydrophilus the anterior and posterior ends develop first, and in Hydrophilus the procephalic lobes originate independently of the rest of the band. In the Aphides, also, according to Will, these lobes arise independently, afterwards uniting with the primitive band.
Fig. 512.—Diagrams illustrating the movements and envelopes of the embryo of Xiphidium: D, the stage of the shortened embryo on the the dorsal yolk. E, embryo returning to the ventral surface. F, embryo nearly ready to hatch; ch, chorion; b. lc, blastoderm membrane; sr, serosa; ind1, outer indusium; ind2, inner indusium; ind2 + am, inner indusium and amnion fused; am, amnion; ind1 c, cuticle of the inner indusium; ind2 s, granular secretion of the inner indusium; am. s, amniotic secretion; v, yolk; cl, columella; gb, primitive band.
Division of the embryo or primitive band into body-segments.—Meanwhile the primitive band grows at the expense of the yolk, spreading out more and more over its surface, until in certain cases (Coleoptera, Diptera, Siphonaptera, and Trichoptera) it lies like a broad ribbon over the yolk, so that the two ends nearly meet on the dorsal side. By this time it becomes divided by transversely impressed lines into segments, which correspond to those of the larva and adult. The first of these segments is divided into two broad and flaring flaps, which are called the procephalic lobes. It becomes the antennal segment.
Fig. 513.—Two stages in the spreading of the indusium. A, lateral view of egg just after the arrival of the embryo on the dorsal yolk. B, lateral view of the egg with the indusium nearly reaching the poles. C, same egg seen from the dorsal surface.
The mouth (stomodæum) now develops, and is situated at the anterior,[[82]] and the rectum (proctodæum,) at the posterior pole, or end of the primitive band.
Fig. 514.—Median section of the egg of Anurida maritima: do, “micropyle”; bld, blastoderm.—This and Figs. 511–513, after Wheeler.