The method is tedious, and the migrations of the primordia on both sides of a single embryo are not always synchronous. After 40 h the primordium is far posterior, small, and indistinct. In addition to these general features, and novel ones that characterize particular stages, there are several important developments during the pharyngula period that we now outline below, and then, following the same order of presentation and setting each topic off into a paragraph of its own, we add details in the stage descriptions. 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.