|THE STAGES OF EMBRYOGENESIS Select image to enlarge in new window||
The Stages of Embryogenesis page 2 | 3 | 4
Stage 14: The beginning of head involution leads to definitive changes in the head region.
Mid-dorsally, the dorsal ridge starts sliding anteriorly, thereby creating a fold (dorsal fold, df)
into which the dorsal head epidermis disappears. The clypeolabrum retracts. As the gnathal buds move anteriorly and ventrally, the deep furrows that had formerly separated these structures flatten. Dorsal closure progresses.
Stage 15: By the end of stage 15, dorsal closure is completed. The amnioserosa as invaginates and subsequently degenerates. The dorsal fold has moved over about half of the dorsal procephalon. As the head structures are displaced interiorly, the segments of the trunk expand and the intersegmental furrows become increasingly shallow.
Stage 17: By stage 17, head involution is completed and the embryonic surface has reached its final, larval morphology. During the course of stage 17, the cuticle gets thicker and cuticle specializations, such as ventral denticles, dorsal trichomes, basiconical sensilla (bs) trichoid sensilla (sh),
and the cephalopharyngeal skeleton secreted by the involuted head epidermis, become visible. (A8, A9) Abdominal segments 8 and 9 (amx) antennomaxillary complex; (ap) anal plate; (cl) clypeolabrum; (ps) posterior spiracle; (T1) thoracic segment 1.