Stage 6

Key events: onset of gastrulation, formation of ventral and cephalic furrow, dorsal shift of pole cells

Stage 6 lasts approximately 10 min only (2:50-3 h). It is the stage of early gastrulation, in which the major morphogenetic movements start:

  1. The invagination of the mesoderm and the anterior endodermal primordium is initiated.
  2. The blastoderm cells at the posterior pole shift their position to form a dorsal plate underneath the pole cells. This is the onset of the germ band elongation.
  3. The cephalic furrow becomes visible at a position corresponding to about 65% egg length (0% egg length = posterior).

Stage 6 begins with the formation of the ventral furrow from which mesoderm and anterior endoderm originate. The ventral furrow is initiated as a median longitudinal cleft that extends between 20% and 70% egg length, along the ventral embryonic midline; over a period of approximately 10 min the ventral furrow will extend further by incorporating additional cells at its anterior and posterior tips, until it extends between 6% and 85% egg length. The ventral furrow forms as a result of cell shape changes which affect an area about 12 cells in width centered on the ventral midline. A total of about 800 cells will become internalized through the ventral furrow: 730 cells represent the mesoderm primordium and 70 cells represent the anterior endodermal midgut primordium.

At the posterior pole of the embryo, a cell plate that carries the pole cells shifts dorsally. This cell plate is flanked anteriorly by a prominent ridge of blastoderm cells, which becomes more and more striking as germ band elongation proceeds.

At the same time, other shape changes become evident within the dorsal half of the embryo; these preceed the formation of the dorsal folds in stage 7.

The cephalic furrow forms at the same time as the ventral furrow. It first becomes visible as a lateroventral slit at about 65% egg length. Later, it extends transversely from the dorsal midline, at about 60% egg length, to the ventral midline at about 75% egg length.

Media list
Stage 6, invagination (0.5 MB)..........additional information
Stage 6, sections..........additional information
Stage 6, in vivo (0.9 MB)

Genes discussed
Gene product - Domains
twist (twi)
transcription factor - bHLH
required for mesoderm development, high Twi levels block formation of visceral mesoderm and heart and induce somatic myogenesis