Embryonic development of the jumping bristletail Pedetonutus unimaculatus Machida, with special reference to embryonic membranes (Hexapoda: Microcoryphia, Machilidae)

Abstract

In the machilid Pedetonutus unimaculatus, a germ disc is formed by the aggregation and proliferation of cells within a broadly defined embryonic area. Cells adjacent to the embryonic area form the serosal fold that grows beneath the embryo. Then the embryonic margin is extended to form a cell layer or amnion that lies between the embryo and serosal fold. Thus, an amnioserosal fold is formed by the addition of the amnion to the serosal fold. Serosal cells cover the entire surface of the egg and begin to secrete a serosal cuticle. Soon the amnioserosal fold is withdrawn, and the embryo is exposed to the egg surface. The spreading amnion replaces the serosal cells that finally degenerate through the formation of a secondary dorsal organ. In the areas of amnion anterior and lateral to the embryo, yolk folds form and encompass the embryo. The amnion is a provisional dorsal closure and never participates in the formation of the definitive one. The amnioserosal fold of the Microcoryphia appears to have the functional role of secreting a serosal cuticle beneath the embryo. This fold of the Microcoryphia may be regarded as an ancestral form of the amnioserosal folds of the Thysanura-Pterygota. the yolk folds may appear to be passive transformation of the yolk mass linked to positioning of the growing embryo within the egg. There is no evidence that the yolk folds and the cavity appearing between them in the Microcoryphia are homologous to the amnioserosal fold and amniotic cavity in the Thysanura-Pterygota. The yolk folds appear to be one of the embryological autapomorphies in the Microcoryphia. © 1994 Wiley-Liss, Inc.

Embryogenesis of the dipluranLepidocampa weberiOudemans (Hexapoda: Diplura, Campodeidae): Formation of dorsal organ and related phenomena

“…Machida and Ando, 1998), as a provisional dorsal closure replacing the degenerating serosa. This interpretation strongly supports the speculation of Machida et al (1994a) and Machida and Ando (1998) that the principal and primary role of the amnion is in provisional dorsal closure. Machida and Ando (1998) regarded the amnion as an autapomorphy of the Ectognatha.…”

“…Johannsen and Butt, 1941). However, Machida et al (1994a) and Machida and Ando (1998) discussed the amnion of Archaeognatha in the light of the evolutionary transition of functional specialization of the embryo proper and embryonic membranes in Atelocerata (ϭ Myriapoda ϩ Hexapoda). They deduced that the principal role of the amnion is simply as a provisional dorsal closure and that the acquisition of the amnion in Archaeognatha, in which formation of the embryonic membrane fold (serosal or amnioserosal fold) is yet to be positively linked with the production of the amnion, may be assumed as the "preadaptation" to the amnioserosal fold-amniotic cavity system shown in the Dicondylia.…”

“…In light of the timing of the formation of the cuticular egg envelope, the Diplura may be akin to the Archaeognatha, which are regarded as the most primitive Ectognatha. In the Archaeognatha the cuticular egg envelope (serosal cuticle) is secreted just before the start of serosal movement and concentration leading to secondary dorsal organ formation at the germ disc or early germ band stage (Machida et al, 1994a).…”

Hold the germ cells, I'm on duty

“…Embryogenesis takes 72 hours in T. ni, and up to 13 days in C. floridanum, so that embryogenesis of the wasp takes place during both embryonic and larval development of the host. C. floridanum cleavage is holoblastic (found in hexapods only among Collembola and parasitic Hymenoptera, (10,(25)(26)(27)(28) ), unlike the syncytial cleavage characteristic of early embryogenesis in most insects. The first cleavage produces two diploid cells of roughly equal size, and the polar bodies, instead of disintegrating, become incorporated into a ''polar cell''.…”

Ultrastructure, development, and homology of insect embryonic cuticles

“…In true insects, the extraembryonic amnioserosal fold covers the embryonic cells, which hence do not participate in cuticular egg envelope secretion. This is true even for Archaeognatha, the basalmost insect clade, in which the amnioserosal folds do not fuse completely (Machida et al, 1994). The only possible exception is Thermobia domestica, whose amnioserosal folds never fuse, leaving the amniotic cavity open, and whose embryo participates in secretion of the cuticular egg envelope (Woodland, 1957;Machida and Ando, 1998).…”