At stage 11, the parts of the foregut primordium giving
rise to the proventriculus (pv) and the adjacent esophagus (es) have invaginated. They form the stomodeum. The primordia of the salivary
glands (sg) invaginate from the ventral labial segment. The posterior
midgut rudiment (pmg), after its cells performed the third
postblastoderm division, has lost its epithelial characteristics and now
forms a solid cluster of rounded cells equal to those of the anterior midgut
rudiment (amg), Four spherical evaginations, which will form the
Malpighian tubules (mp), appear in the hindgut primordium
close to its boundary to the posterior midgut rudiment (see Skaer, this
During stage 12, a significant
reorganization of the midgut rudiments takes place. First, both anterior
and posterior midgut rudiments stretch in the longitudinal axis and become
bilobate. Then, during germ-band retraction, the midgut rudiments approach each
other and finally fuse at about 50% egg length.
The midgut primordium of a
stage 13 embryo (mg) forms two longitudinal plates of epithelial
cells flanking the central yolk. These plates will spread in the
transversal axis and eventually
[stage 15] fuse in the ventral and dorsal midline,
thereby forming a closed chamber that encloses the yolk. Anteriorly and posteriorly,
the plates abut the presumptive proventriculus (pv) and hindgut (hg), respectively. The hindgut has adopted its characteristic sigmoid shape; the
Malpighian tubules (mp) have elongated considerably.
Anteriorly, the pharynx (ph) starts to invaginate. The gnathal
segments, part of which will later form the atrium, have moved anteriorly.
(ap) Anal pads; (ms) mesoderm; (sm) somatic
mesoderm; (vm) visceral mesoderm.