Key events: completion of gastrulation, pole cells in a pocket

Stage 7 lasts for about 10 min (3:00-3:10 h). During this stage, gastrulation is completed: the endodermal primordia of the anterior and posterior midgut, and the primordium of the hindgut invaginate. The foregut invaginates at stage 9.





At the posterior egg pole, in stage 6, a discoid plate of approximately 150 cells had shifted its position towards dorsal, carrying the pole cells on it. Stage 7 begins when the cell plate has attained a horizontal orientation on the dorsal egg surface.

The cell plate tilts inward anteriorly and eventually forms a pocket that contains the pole cells. The cells immediately posterior to this deepening depression sink inwards along the midsagittal plane, to form a deep groove that is continuous with the ventral furrow. The entire structure – the pocket containing pole cells and the neck of the pocket – is called the proctodeal invagination. Stage 7 ends when the anterior wall of the proctodeal invagination starts to move cephalad.

During stage 7 the anterior midgut primordium further invaginates ventrally (stomodeal invagination) at about 85% egg length (0% egg length = posterior pole) and is continuous with the ventral furrow, whose anterior end it forms.

In a lateral view, three folds are clearly visible at stage 7, which diverge in a dorso-ventral direction: the cephalic furrow, the anterior transverse furrow and the posterior transverse furrow. The latter are superficial, short-lived structures which change their position, while advancing anteriorly as germ band elongation progresses, whilst the cephalic furrow is a deep invagination that will persist until to stage 9. Most of the cells forming the transverse furrows comprise the primordium of the amnioserosa.

Cells that remain at the surface of the embryo after invagination of the ventral furrow (including endodermal primordia), constitute the ectoderm and the amnioserosa. Ectodermal cells amount to approx. 3700 cells, of which 1000 are located in front of and 2700 behind the cephalic furrow.