It has been widely received that one of Gregor Mendel ’s most important contribution to the history of genetics is his novel work on developmental information (for example, the proposal of the famous Mendelian ratios like 1:2:1, 3:1, and 9:3:3:1). This view is well evidenced by the fact that much of early Mendelians’ work in the 1900s focuses on the retrodiction (i.e. the re-analysis of the pre-exist data with Mendel ’s approach). However, there is no consensus on what Mendel meant by development (Entwicklung). Nor is there an agreement on the interpretation of Mendel ’s laws of developmental series (Entwicklungsreihe). This chapter revisits Mendel ’s notions of development and developmental series. First, I argue that Mendel ’s use of development is greatly influenced by Gärtner ’s. Second, I show Mendel ’s work on developmental series are novel and important for its new ways of experimentation, conceptualisation, and analysis. Third, I argue that Mendel ’s laws of developmental information were not about heredity.