(A) During gastrulation a hollow sphere of cells (left) is transformed into an elongated embryo (right). This starts with cells on the dorsal (upper) side of the embryo rolling over a structure called the blastopore (red circle) and moving to the inside of the sphere. This reduces the size of the blastopore. Cell-autonomous convergence forces (purple) then cause the sphere to elongate, leading to the formation of the anterior–posterior axis. (B) The process by which tissue elongates along the anterior-posterior (AP) axis, and becomes narrower along the medio-lateral (ML) axis, is called convergent extension. (C) Shook et al. discovered that a process called convergent thickening – which involves the tissue becoming thicker in the direction at right angles to the convergent extension – is also important during gastrulation. (D). Sketch showing how the convergence force (y-axis) increases through gastrulation, and then plateaus (during early neurulation) before increasing again (during late neurulation).