Dry titania layers on air-oxidized titanium substrates have been found to be active enough to cause apatite to be deposited in Kokubo's simulated body fluid (SBF) in narrow confined spaces, such as those in narrow grooves and thin gaps. Such in vitro apatite deposition is the basis of the GRAPE(®) technique. The aim of the present study is to determine why GRAPE conditions favor apatite deposition when laminar SBF flow (at 0.01-0.3 ml/min) passes through a shallow channel (0.5 mm) between a pair of titanium substrates each with a dry layer of titania. Assessing the factors that control the heterogeneous nucleation process led to the proposal of the working hypothesis that there are nucleation pre-embryos, ion assemblies that can be stabilized to form embryos, on the titania layer but that they are removed by the SBF flow. Specimens were subjected to different combinations of processes. One combination was that titania layers were exposed to still or flowing SBF, and the other was that half of a specimen, the inlet or outlet side, was exposed to still or flowing SBF with the other half being covered. The surface morphologies of the specimens were then compared in detail. The conclusion was that exposure to still SBF for 2 days before exposure to flowing SBF was required for apatite to be deposited. Some complicated apatite deposition modes were observed, e.g., apatite was deposited even on areas unexposed to still SBF. All of the results were successfully interpreted using the working hypothesis. The conclusion was that the GRAPE(®) technique depends on the confined space holding pre-embryo and embryo assemblies.