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Appendix I              Stage Conversions for Chick embryos
HH STAGE            AGE                        IDENTIFYING FEATURES
            1                      0 h                        Pre-primitive streak (embryonic shield)
            2                      6-7 hr                        Initial primitive streak, 0.3-0.5 mm long
            3                      12-13 hr                        Intermediate primitive streak
            4                      18-19 hr                        Definitive primitive streak, ±1.88 mm long
            5                      19-22 hr                        Head process (notochord)
            6                      23-25 hr                        Head fold
            7                      23-26 hr            /1da            1 somite; neural folds
            7 to 8-                    ca. 23-26 hr            1-3 somites; coelom
            8                      26-29 hr                        4 somites; blood islands
            9                      29-33 hr                        7 somites; primary optic vesicles
            9+ to 10-                  ca. 33 hr                        8-9 somites; anterior amniotic fold
            10                     33-38 hr                        10 somites; 3 primary brain vesicles
            11                     40-45 hr                        13 somites; 5 neuromeres of hindbrain
            12                     45-49 hr            /2da            16 somites; telencephalon
            13                     48-52 hr                        19 somites; atrioventricular canal
            13+ to 14-                 ca. 50-52 hr            20-21 somites; tail bud
            14                     50-53 hr                        22 somites; trunk flexure; visceral arches I and II, clefts 1 and 2
            14+ to 15-                 ca. 50-54 hr            23 somites; premandibular head cavities
            15                     50-55 hr                        24-27 somites; visceral arch III, cleft 3
            16                     51-56 hr                        26-28 somites; wing bud; posterior amniotic fold
            17                     52-64 hr                        29-32 somites; leg bud; epiphysis
            18                     3 da                        30-36 somites extending beyond level of leg bud; allantois
            19                     3.0-3.5 da            37- 40 somites extending into tail; maxillary process
            20                     3.0-3.5 da            40-43 somites; rotation completed; eye pigment
            21                     3.5 da                        43-44 somites; visceral arch IV, cleft 4
            22                     3.5-4.0 da            Somites extend to tip of tail
            23                     4 da                        Dorsal contour from hindbrain to tail is a curved line
            24                     4.5 da                        Toe plate
            25                     4.5-5.0 da            Elbow and knee joints
            26                     5 da                        1st 3 toes
            27                     5.0-5.5 da            Beak
            28                     5.5-6.0 da            3 digits, 4 toes
            29                     6.0-6.5 da            Rudiment of 5th toe
            30                     6.5-7.0 da            Feather germs; scleral papillae; egg tooth
            31                     7.0-7.5 da            Web between 1st and 2nd digits
            32                     7.5 da                        Anterior tip of mandible has reached beak
            33                        7.5-8.0 da            Web on radial margin of wing and 1st digit
            34                     8 da                        Nictitating membrane
            35                        8.5-9.0 da            Phalanges in toes
            36                     10 da            Length of 3rd toe from tip to middle of metatarsal joint = 5.4±0.3mm; length of beak from anterior angle of nostril to tip of bill = 2.5mm; primordium of comb; labial groove; uropygial gland
            37                     11 da                        Length of 3rd toe = 7.4±0.3mm; length of beak = 3.0 mm
            38                     12 da                        Length of 3rd toe = 8.4±0.3 mm; length of beak = 3.1 mm
            39                     13 da                        Length of 3rd toe = 9.8±0.3 mm; length of beak = 3.5 mm
            40                     14 da                        Length of beak = 4.0 mm; length of 3rd toe = 12.7±0.5 mm
                                                41            15 da            Length of beak from anterior angle of nostril to tip of upper bill = 4.5 mm; length of 3rd toe = 14.9±0.8 mm
            42                     16 da                        Length of beak = 4.8 mm; length of 3rd toe = 16.7±0.8 mm
            43                     17 da                        Length of beak = 5.0 mm; length of 3rd toe = 18.6±0.8 mm
            44                     18 da                        Length of beak = 5.7 mm; length of 3rd toe = 20.4±0.8 mm
                                              45            19-20 da            Yolk sac half enclosed in body cavity; chorio-allantoic membrane contains less blood and is "sticky" in living embryo
            46                       20-21 da                        Newly-hatched chick