The distal epithelial fold capping the bud, that developed from the apical ectodermal ridge, now expands into the blade of the fin proper and strengthening actinotrichia appear. At the same time a circulatory channel appears as a continuous loop at the base of the enlarging fin, i.e. its position is proximal to the actinotrichia but distal to the cartilage. This channel will develop both as artery and vein. The subclavian artery takes the blood outward into the fin from the paired portion of the dorsal aorta posterior to the last aortic arch. The subclavian vein returns the blood again, connecting proximally near the origin of the common cardinal.
Prominent changes occur in the pharyngeal region.
Then, especially during the last 12 hours of embryogenesis, a dramatic repositioning of the mouth occurs. Jaw morphogenesis moves the position of the mouth anteriorwards, and just at the end of the hatching period the mouth protrudes beyond the eye, where it gapes wide open. Rapid jaw and mouth morphogenesis will continue in the early larva. Examining cartilage development in the jaw primordia is also useful for staging, especially during the last part of the hatching period. On the other hand the jaw cartilages develop ahead of the branchial cartilages.