Figure 1(b) shows 3D reconstruction of a zygote stage (E0.5) embryo, and the cross-section view in Fig. 1(c). Besides ZP, zygote, and 2PB, three relatively dark areas (DA1, DA2, and DA3) are also clearly visible. Two of them lie in the zygote and the third in the 2PB. 3D reconstruction of three dark areas segmented from the image stack is shown in Fig. 1(d). By the size, shape, and location, the dark areas were ascertained to be the nuclei. To confirm this, DNAs of the same embryo were stained with Hoechst 33342 and scanned with two-photon laser scanning microscopy (TPLSM). Figure 1(e) shows 3D reconstruction of three nuclei [from left to right, three nuclei are the nucleus of the 2PB (2PBN), female pronucleus (FPN), and male pronucleus (MPN), respectively].26 Owing to TPLSM’s capability of 3D quantitative measurement, a triangle, whose vertices are fluorescence centers of three nuclei, was characterized. The lengths of three sides were 19.1, 44.4 and 26.0 µm, respectively, shown in Fig. 1(e). In comparison, a similar triangle was also measured with FF-OCT. Its vertices were 3D geometry centers of the three dark areas in FF-OCT image, shown in Fig. 1(d). The corresponding side lengths of the triangle were measured to be 19.1, 44.0, and 25.7 µm, perfectly coincident with the TPLSM’s measurements with an accuracy of FF-OCT resolution. Furthermore, FF-OCT image gave essentially the same volumes of three dark areas as those of corresponding nuclei in TPLSM image. Therefore, we may affirm that the dark areas in the FF-OCT image are no other than the nuclei. On the other hand, chromatin is structurally loose during interphase, and its size is far below the wavelength of light. Therefore, the nucleus, whose main part is chromatin, has a low level of scattering signal and shows relatively dark in FF-OCT image. As different organelles have different scattering coefficients and thus give different grey-level signals, FF-OCT is also a functional imaging technique capable of distinguishing functional areas in a cell without any dye labeling.