The fins begin to form. As the buds develop, an apical ectodermal ridge becomes prominent at their tips. Pigment cells differentiate. They are easy to see and we use them for staging landmarks. The heart begins to beat just at the onset of the period, and forms well delineated chambers.
Blood begins to circulate though a closed set of channels.
The others develop rapidly near the end of the period. The blood flows into each side of the head from the anterior two arches via the carotid artery and returns via the anterior cardinal veins. At a point along the tail the channel makes a smooth ventrally directed 180 degree turn to form the caudal vein, returning the blood to the trunk. The vein continues in the posterior trunk as the unpaired median axial vein, lying just ventral to the dorsal aorta. Just posterior to the heart, the vein then splits into the paired left and right posterior cardinal veins.