Lastly, we used OCT to investigate how a ciliated surface transitions from the absence of flow to the presence of flow. Like other organs, ciliated surfaces undergo a developmental process. The development of coordinated ciliary flow in Xenopus embryos is known to occur over approximately 24 h and to involve the interplay between tissue patterning and hydrodynamic signaling.14 In spite of having been extensively studied from a molecular perspective, however, to our knowledge, the flow speed during this period has not yet been described in a longitudinal and quantitative manner. Thus, we quantified the average tangential speed during the onset of flow during 24 to 50 h, corresponding approximately to Nieuwkoop–Faber (NF) developmental stages 21 to 34 in X. laevis.15 We imaged nine embryos over this period, with measurements taken every 2 to 4 h [Fig. 3(a), Video 2]. For each measurement, the embryo was immobilized with benzocaine and suctioning for the duration of imaging (), and then immediately washed and placed in a reservoir of clean MR. Aggregating the data, we generated a normative curve [Fig. 3(b)] showing the average flow plus or minus one standard deviation (STD). Embryos began the process with an average flow speed of (STD) at NF stage 21, and increased to by approximately stage 34. Notably, 2/9 embryos failed to develop a sustained flow during this period.