Fig. 14.

Model for the evolution of axis formation in vertebrates. Highly generalized schematic diagrams of different vertebrate embryos during early (upper panels) and middle (lower panels) gastrulation. In the basal vertebrates (left panels; vegetal views), gastrulation initiates at the organizer with induction by the Nieuwkoop center (high/early Nodal signaling). The initial internalization movements are through involution. By mid-gastrulation, ingression of mesendoderm progresses around the vegetal cells (yolk plug) and forms the nascent blastopore. Involuted dorsal mesendoderm undergoes convergent extension via mediolateral cell intercalation under the control of Wnt/PCP signaling. During the evolution of amniotes (middle panels; top/dorsal views), eggs increased in size and yolk content and began to undergo meroblastic cleavage. Organizer induction by Nieuwkoop center molecules is retained in early gastrulation. Ingression proceeds through the horizontal slit of the blastoporal plate/blastopore and does not circumferentially envelop the non-cleaving yolk (not shown). In the evolution of modern birds and mammals, gastrulation initiates with ingression at the primitive streak, which would be homologous to the later ventrolateral blastopore in ancestral forms. The organizer (Hensen’s node) is induced later by Nodal signaling from the middle primitive streak. The heterochrony in the pattern of gastrulation morphogenesis and organizer formation could result from several main events; the hypoblast/anterior endoderm segregating from the epiblast as opposed to forming from cleaving vegetal cells, the loss of early organizer induction, the apparent emergent behavior of polonaise-like movements leading to primitive streak formation and the relatively later induction of Hensen’s node by the primitive steak