Martensite is a heterogeneous, nucleation-controlled, and autocatalytic transformation with strong industrial relevance due to its ability to impart the properties of engineering steels during production, manufacturing, and use. In this work, a formal model of the spread of martensite nucleation permitted obtaining the cumulative number density of embryos in a bulk, polycrystalline Fe-31 wt%Ni-0.02 wt%C alloy based on standard metallography measurements. The result shows that the density of embryos available to initiate the reaction in a bulk; polycrystalline Fe-31 wt%Ni-0.02 wt%C alloy is significantly smaller than the density of pre-existent martensite embryos available in a similar alloy constituted of isolated small particles or the particulate. In a polycrystal, the present authors have previously proposed that embryos that do initiate the transformation originate from a volume of influence of the grain boundaries. Therefore, in addition to the usual concept of embryo potency, a new concept, namely, embryo association must be introduced. In a polycrystal, fewer but better localized, i.e. embryos associated with the grain boundaries, which are located within the volume of influence of the grain boundaries, initiate martensite transformation larger number of embryos which may exist within the grains. According to this view, this difference in the embryo association in the polycrystal and in the particulate is the main factor contributing to the difference in the martensite density of embryos available to propagate the transformation observed in this work.