Fig. 1

Proposed models describing morphogenetic events that contribute to the bending of the animal-vegetal axis of the spiralian embryo. These events reposition the mouth to the future ventral side of the embryo and closer the animal (future anterior) pole. a-c Earlier model suggesting that the vegetal pole and site of gastrulation (where the blastopore and mouth form) is displaced by differential proliferation of post-trochal D quadrant progeny (mainly 2d progeny) on the posterior dorsal side of the embryo [32]. d-f Model proposed here based on data from C. fornicata where axial bending is driven by ventral displacement of the animal pole through rearrangement and flattening of 1q² (i.e., 1a¹-1d¹) and 1d¹ (i.e., 1d¹²¹ and 1d¹²²) progeny. While these two models are not mutually exclusive, the data reported here revealed the latter process in C. fornicata, and indicate that these cells are repositioned towards the dorsal side of the head to form an expansive, provisional epithelium, which is subsequently lost before hatching. Cells are labeled following the nomenclature of Conklin [40], and as described in the text. bp, blastopore; A, animal pole; st, stomodeum; V, vegetal pole