Gastrulation - Autonomous Extension in Exogastrulae

To test whether secondary mesenchyme cells are actually needed for archenteron elongation, we can perform several experiments. First, it is possible to produce "exogastrulae", in which the archenteron evaginates, rather than invaginating. In this case, the archenteron does elongate, although it is roughly 2/3 of its normal final length.

From Hardin and Cheng (1986), with permission.