Owing to the important role of homogeneous nucleation in grain refinement of rapidly solidified alloys, a detailed molecular dynamics simulation is performed to investigate the incubation of embryos and their evolution into nuclei during the isothermal crystallization of liquid Al droplets. Using the cluster type index method (CTIM) based on Honeycutt-Andersen (H-A) bond-type indices, various fcc critical nuclei formed during isothermal crystallization are distinguished from numerous fcc embryos through reverse tracking of atomic trajectories, relying on the structural heredity of fcc single-crystal clusters. The results show that nuclei first appear in the shell region of Al droplets with a critical size (n_{\mathrm{c}}) ranging from 2 to 100 atoms at an undercooling of ΔT ≈ 0.41T_m (T_m is melting point). Both the steady-state nucleation rate (I₀) and the average critical nucleus size ({\overline{n}}_{\mathrm{c}}) in the shell are higher than those in the core region. Visual analysis of the geometry of critical nuclei reveals that most are non-spherical, and the liquid-solid interface is not a simple fcc-liquid dual-phase configuration, but rather a multi-phase structure involving fcc-liquid and hcp components. Compared with the nucleation in Al bulk, a longer average nucleation incubation time ({\overline{\tau }}_{\mathrm{c}}) of critical nuclei is observed in Al droplets, with {\overline{\tau }}_{\mathrm{c}} in the shell region being longer than that in the core. When {\overline{\tau }}_{\mathrm{c}} is divided into the average incubation time of embryos ({\overline{\tau }}_{\mathrm{e}}) and their average effective growth time ({\overline{\tau }}_{\mathrm{g}}^{\mathrm{e}\mathrm{f}\mathrm{f}}), it is determined that {\overline{\tau }}_{\mathrm{g}}^{\mathrm{e}\mathrm{f}\mathrm{f}} is considerably longer than {\overline{\tau }}_{\mathrm{e}} in both Al droplets and Al bulk. For the four modes of nucleation, i.e., (I) embryo incubation and subsequent effective growth, (II) only effective growth of embryos, (III) direct nucleation after embryo incubation, and (IV) direct transformation from liquid atoms, a tracking analysis of atomic trajectories reveals that few critical nuclei are formed directly from liquid atoms. In contrast, most critical nuclei undergo both embryo incubation and effective growth, and these exhibit the largest {\overline{n}}_{\mathrm{c}}. Moreover, the incubation time ({\tau }_{\mathrm{e}}) of embryos has little effect on {\overline{n}}_{\mathrm{c}} of critical nuclei, whereas a large {\overline{n}}_{\mathrm{c}} typically requires a long effective growth time ({\tau }_{\mathrm{g}}^{\mathrm{e}\mathrm{f}\mathrm{f}}) of embryos during isothermal crystallization.