13 is short, lasting for about 1 hour (9:20-10:20 h). It is initiated
by the completion of germ band shortening, when the prospective anal pads
occupy the posterior egg pole and ends with the beginning of head involution.
Cells of the midgut epithelia expand over the yolk sac
to make contact along the ventral and dorsal midline and eventually fuse.
This process takes quite a long time to be complete, extending into stages
14 and 15.
Three different regions can be distinguished in the foregut, the pharynx, consisting of the pharyngeal roof and hypopharyngeal lobes, the prospective esophagus and the proventriculus. The three invaginations of the stomatogastric nervous system have disappeared, their cells becoming distributed within the clypeolabrum.
The two developing salivary gland lobes join to form
a common midsagittal duct behind the hypopharyngeal lobes at the beginning
of stage 13; the salivary gland duct will be displaced towards the foregut
as the labial appendages move frontalward during mouth involution. The
salivary glands diverge bilaterally from the common duct, extending between
the epidermis and the ventral cord beneath the midgut.
The developing central nervous system consists of the
well differentiated ventral cord and the supraoesophageal ganglion. Neuromere
organization is clearly distinguishable within both the ventral nerve
cord and the suboesophageal ganglion. Ventral cord and suboesophageal
ganglion extend from the tip of the hypopharyngeal lobe to the region
immediately ventral to the anus, and will maintain this length until stage
15, when ventral cord condensation sets in. The primordia of the optic
lobes have become integrated into the posteroventral region of the supraoesophageal
ganglion, although their cells can still be distinguished cytologically.
Fibre connectives and commissures linking the different neuromeres are
formed. Progenitors of sensory cells, including the primordia of the antenno-maxillary
complex, appear in the epidermis of maxilla, mandible and procephalic
lobe. At the end of stage 13 muscle cells become apparent, inserting at
incipient apodemes of the lateral epidermis.
Stage 13, in vivo (1.2 MB)
Stage 13, sagittal sections (0.8 MB)..........additional information
Stage 13, virtual microscope, sagittal sections..........additional information
Stage 13, anti-Crumbs..........additional information