Haeckel's Biogenetic Law and the Land Plant Phylotypic Stage
Haeckel’s 150-year-old biogenetic law has been refuted by embryological studies of metazoans (predominantly vertebrates). However, modern empirically viable variants exist, such as the concept of the phylotypic stage that emerged predominantly from the study of metazoans. We briefly review the history of the biogenetic law and the evidence for the metazoan phylotypic stage, we explore whether a phylotypic stage exists for the land plants by comparing the embryogenesis of diploid sporophytes across nonvascular and vascular lineages, and we examine whether homologies exist for haploid gametophytes. If homology is defined as “the same structure(s) achieved by the same developmental patterning(s),” we fail to find a single phylotypic stage for land plant sporophytes or gametophytes based on morphological criteria. We speculate that land plant embryogenesis has undergone three major evolutionary transformations (heralding the appearance of bryophytes, pteridophytes, and seed plants) that correspond with evolutionary transformations in the preceding ancestral phylotypic stage.