Remaining questions related to the hourglass model in vertebrate evolution

Despite the phenotypic divergence of early embryogenesis among vertebrates (e.g., the wide variety of cleavage and gastrulation patterns), all species converge into phenotypically similar mid-embryonic stages (particularly pharyngula embryos, which show the typical anatomical features of vertebrates, such as the pharyngeal arch), and evolutionary divergence occurs again thereafter. This observation coincides well with the recently supported developmental hourglass model; however, little is known about the nature of this conserved pharyngula period in vertebrates, and it is unclear why this mid-embryonic period has been conserved. By highlighting recent molecular-based studies, this review focuses on known information and what should be known on this topic, with a focus on vertebrate pharyngula embryos.