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Table 1 Hallmarks of each developmental stage in O. vulgaris

From: A practical staging atlas to study embryonic development of Octopus vulgaris under controlled laboratory conditions

Stage Characteristic
Stage 0 Cleavage
Stage I Morula (advanced cleavage)
Stage II Blastula; disk flatter
Stage III Onset of epiboly
Stage IV Formation of the germinal disk, as wide as the yolk
Stage V Epiboly reaches 1/4th of the yolk
Stage VI Epiboly reaches 1/2nd of the yolk
Stage VII.1 Epiboly reaches 3/4th of the yolk
Visible thickening of placodes starts
Stage VII.2 Embryo completed first reversion
Primordia of eyes, mouth, mantle and arms clearly visible
Stage VIII Mouth and eye invagination
Mantle elevated and embryo thicker
Funnel pouches visible in lateral view
Stage IX First eye pigmentation (yellowish)
Primordia more prominent
Funnel tube rudiments are distinct
Contraction of yolk envelope evident
Stage X Mantle flat, no depression in the middle
Eye vesicles sticking out with light orange retina (“saddle” shape)
Funnel tube rudiments fuse at the ventral margins
Stage XI Mantle tilted
Funnel tube rudiments bended towards midline
Arm buds ‘elevated’ from yolk
Stage XII.1 Mantle thicker and covers 1/2nd of gills
Funnel tube rudiments start to form a tube ventrally
First suckers recognizable on the posterior side
Stage XII.2 Mantle bowl-shaped
Funnel formed siphon at ventral extremity
Stage XIII Mantle is bigger
Formation of the funnel complete
Arms elongated and pointed with (3) prominent suckers
Stage XIV Mantle as wide as long and cube-shaped, covers gills completely
Heartbeat starts
Embryo and yolk have equal size
Stage XV.1 Mantle completely covers ventral margin of funnel
Inner yolk strongly constricted (connection inner and outer yolk sac very thin)
Appearance of two chromatophores laterally from the funnel
Stage XV.2 Mouth encircled by anterior arms
First chromatophores on posterior mantle appear
Stage XVI Mouth completely covered by arm crown
Yolk size 1/3rd of embryo + yolk
Few chromatophores on anterior mantle evident
Stage XVII Chromatophores darker and more numerous
Stage XVIII.1 Yolk size 1/4th of embryo + yolk
Posterior chromatophores darker and chromatophores appear next to the eye
Embryo more active in egg (mantle contraction)
Stage XVIII.2 Anterior chromatophores darker. Chromatophores react to light stimulus
Stage XIX.1 Eyes tilted and covered with iridophores
Yolk size 1/6th of embryo + yolk
Embryos react to mechanical stimulus
Pigmentation of ink sac
Stage XIX.2 Embryos completed second reversion
Stage XX.1 Minimal outer yolk sack
Chromatophore expansion and contraction more widely distributed
Stage XX.2 Absence of outer yolk