In insects with an invaginated primitive band, of which the Odonata afford examples, the first rudiment of the primitive band is in the form of a ventral plate of slight extent passing ventrally in the hinder half of the egg, in whose posterior section a process of invagination (Fig. 518, A, kh), soon occurs. The cavity of this invagination is the first indication of the amnion-cavity (Fig. 518, B, ah), while its wall in its thickened ventral part (K′) is concerned in the formation of the primitive band, and, in its dorsal thin part, in the formation of the amnion (B, C, am).
Revolution of the embryo where the primitive band is invaginated.—At first the head-end of the embryo is directed towards the posterior end of the egg, as in dragon-flies (Fig. 518). Also that surface of the primitive band which afterwards faces the ventral, is at first turned towards the dorsal side of the egg. In order to bring the primitive band into the later relations, there must occur the process of revolution, or turning, of the embryo. The somewhat advanced embryo of the Odonata, after the appearance of the head and thoracic appendages, undergoes a rotating motion around its transverse axis, and at the same time turns out of the amniotic cavity (Fig. 520, B). This process is so managed that near the head-region, the amnion and serosa, there closely situated to each other, are fused together, and at this place tear or burst open. Through this rent (a), in the same place in which the original invagination-opening was situated, the amniotic cavity again opens, and through the opening thus formed first the head and then the succeeding segments of the primitive band (Fig. 520, B) pass out, and remain there while the head passes on to the anterior pole of the egg on the ventral side, the embryo thus assuming a position like that of other insects. (Kowalevsky.)
In the parasitic Hemiptera (Pediculina), according to Melnikow, the opening in the membranes near the head remains permanent, and the embryo becomes everted through it, while the yolk, enclosed in the continuous membrane formed by the amnion and serous membrane, forms a yolk-sac on the dorsal surface. The same process occurs in Mallophaga, and also in Œcanthus, as described by Ayers (Fig. 521). Generally as soon as the embryo passes out of the amniotic cavity the latter soon becomes smaller and finally completely disappears.
Fig. 521.—Revolution of the embryo of Œcanthus (diagrammatic): a, fore, b, hind end of egg; am, amnion; d, dorsal, v, ventral side of egg; k, primitive streak; r, dorsal plate (originating by the contraction of the serosa (s)).—After Ayers, from Korschelt and Heider.
As the embryo grows, and the sides grow up and the back closes over, the contents of the yolk-sac are soon taken up and absorbed in the intestinal cavity, which communicates with it.
In Phyllodromia, according to Wheeler, the process of revolution is “hurried through by the embryo from the beginning of the 16th to the end of the 17th day.” Several successive stages are represented in Fig. 522. In the 15th day the embryo still occupies the middle of the ventral surface of the egg. Soon the envelopes (amnion and serosa, as) rupture, an irregular slit being formed, and soon the egg and embryo are as seen in Fig. 522, B, the embryo standing out free from its envelopes on the yolk, and the edges of its dorsal growing walls (b) are distinctly marked. The tail now lies at the caudal end of the egg (Fig. 522, C). By the 17th day the walls have closed in the median dorsal line, and the embryo has grown in length to such an extent as to bring its head to the cephalic pole (Fig. 522, E).
Korschelt and Heider consider, since the primitive band of the chilopod myriopods (Geophilus) is curved in at the middle and sinks into the interior of the yolk, that in insects the invaginated primitive band is the ancestral or primitive one, the overgrown primitive band being derived from it. The overgrown primitive band by its position may also be better insured against certain mechanical attacks, perhaps also against the danger of drying up.
Fig. 522.—Embryo of Phyllodromia, 15 days old; revolution about to begin. The stages in revolution are represented, after the rupture of the amnion and serosa, in A to E, which are from embryos 16, 16½, 16¾, and 17 days old respectively: as, amnion and serosa; s, edge of serosa; b, dorsal growing body-wall; d.o, dorsal organ; x, clear zone covered with scattered amniotic nuclei.—After Wheeler.