Development the sexually-produced pea aphid embryo, under 'winter-like' conditions. (A) At day 15 the antennae (an), mandibles (not visible in this image), maxillae (mx), labium (lb), and thoracic limbs (t1–t3) are visible, as are the germ cells (gc) and bacteriocyte (b). (B) By day 21 the embryo has straightened with considerable growth of the appendages. (C) Nuclear staining of anti-Histone H3, which can be seen as brown spots (arrowhead) on the embryo, reveals cell division at day 21. (D) At day 35 the legs but not the body are longer than at day 21. (E) Cell division is also evident at day 35 with anti-Histone H3 staining (arrowhead). (F) At day 48 dividing cells stained with anti-Histone H3 are again observed (arrowhead). At this stage the embryo is lying in the centre of the egg. (G) At day 63 the embryo begins katatrepsis and has moved to the posterior of the egg, with the amnion (am) in contact with the serosa (s). (H) At day 70 the embryo is curled at the posterior of the egg. The amnion and serosa have fused into a single membrane, here called the amnioserosa. (I) At day 77 the embryo has completed katatrepsis and has a cap of putative aminioserosa at its anterior end. (J) At a slightly later stage another embryo has a reduced "amnioserosal cap". (K) At day 84 dorsal closure has been completed and the embryo has deposited an embryonic cuticle, complete with egg burster (eb). All scale bars are 100 μm long. Embryos are orientated as they would be in the egg, with the anterior of the egg to the left.