The inner of the two layers, called the amnion (Fig. 509, am), is more closely connected with the embryo. The amnion and ectoderm of the primitive band together form a completely closed sac, whose lumen forms the amniotic cavity. Originally connected with the serous membrane, it splits off from the primitive band about the time the appendages begin to bud out, and continues to closely envelop the body and appendages, as seen in Fig. 509. Both of these membranes are, before the time of hatching, either absorbed, or, as in Lepidoptera, retained. The amnion is retained until after hatching in the locust, etc. In certain Coleoptera the serosa is retained, and the amnion is absorbed (Fig. 532), while in Chironomus and the Trichoptera the serosa is absorbed, and the amnion retained, with the egg-shell or chorion. Hence we have eight layers in the winged insects[[81]] during embryonic life:
Fig. 510.—Surface view of fresh serosa from an Œcanthus, treated with acetic carmine; the blastoderm completely formed, × 500: p, polar body; rf, radiating fibres; nls, nuclear substance; nlm, nuclear membrane.—After Ayers.
1. Exochorion. (Remains of the epithelium of the ovarian follicle.) 2. Chorion. (Egg-shell or cuticle secreted in the ovarian follicle.) 3. Vitelline membrane. (Primary egg-membrane. Yolk-skin or membrane.) 4. Serous or outer germ-membrane. (Serosa.) } 5. Amnion or inner germ-membrane. } Derived from the blastoderm. 6. Ectoderm. } } 7. Mesoderm. } Embryo. } 8. Endoderm. } }
In the embryo of Xiphidium and Orchelimum Wheeler has found and described with much detail a membranous structure which he calls the indusium. “The organ,” he says, “appears to have been retained by the Locustidæ, and completely lost by the embryos of other winged insects.” It arises in Xiphidium, as a simple circular thickening of the blastoderm, between and a little in front of the procephalic lobes (Figs. 511, 512, A-E), and afterwards spreads over nearly the whole surface of the egg, leaving the poles uncovered, as in Fig. 513, where it is divided into two further membranes, the inner and outer indusium, the former lying in contact with the amnion. After this the serosa “is excluded from taking any part in the development of the embryo; both its position and function are now usurped by the inner indusium.”
Hence in an egg of the Locustidæ Wheeler distinguishes, passing from within outward in a median transverse section of the egg, the following envelopes:
1. The chorion. 2. The blastoderm-skin-like cuticle secreted by the serosa. 3. The serosa. 4. The outer indusium. 5. A layer of dark granular secretion (probably some urate). 6. The cuticle secreted by the inner indusium. 7. The inner indusium. 8. The amnion. While envelopes 1–7 invest the whole egg; layer 8, the amnion, covers only the embryo.
Fig. 511.—Diagrams illustrating the movements and envelopes of the embryo of Xiphidium: A, after the closure of the amnioserosal folds. B, during the embryo’s passage to the dorsal surface. C, just after the straightening of the embryo on the dorsal surface; ind, indusium afterwards forming ind1, the inner, and ind2, the outer indusium; ch, chorion; sr, serosa; am, amnion; gb, germ-band; v, yolk; bl. c, blastoderm membrane.
Wheeler further suggests that the so-called micropyle of the Collembola (Anurida), which has been homologized with the “dorsal organ” of Crustacea, is a possible homologue of the indusium, as also the “primitive cumulus” of spiders, and the “facette” or “cervical cross” of Pentastomids described by Leuckart and also by Stiles.