Key events: onset of germ band retraction, fusion of anterior and posterior midgut

Stage 12 lasts for about 2 h (7:20-9:20 h). The major event in this stage is the shortening of the germ band. The germ band retracts caudalwards in such a way that the opening of the hindgut comes to be located at the dorsal side of the posterior egg pole; at the same time the width (mediolateral extent) of the germ band increases approximately one and a half times.

Anterior and posterior midgut rudiments can now be clearly seen as two distinct bilateral cell bands that move toward each other, to eventually meet at about 60% egg length (0% egg length = posterior pole) at the end of stage 12. Segmentation becomes very prominent during germ band shortening. One of the consequences of germ band shortening is that the yolk sac is pushed to the dorsal side of the embryo, being covered only by the amnioserosa. Furthermore, the yolk sac retracts from the cephalic regions, flanked by the anterior and posterior midgut anlagen.

At the roof of the develping foregut, three invaginations appear; they will give rise to the stomatogastric nervous system. The advancing tracheal tubules remain open to the outside and start to branch, eventually fusing with each other to form a continuous tube.

During stage 12 the ventral cord separates completely from the epidermis; the first axons in the ventral nerve cord appear.