events: dorsal closure to 80% along the dorsoventral axis, head involution
Stage 14 lasts for 1 hour (10:20-11:20 h). It begins with the onset of head involution and is characterized by the progression of three major morphogenetic events:
i) dorsal closure
ii) closure of the midgut
iii) head involution
After completion of germ band shortening dorsal epidermal cells flatten out and spread dorsally over the amnioserosa. In this manner amnioserosa and epidermis transiently overlap, though the cells of the two layers do not intermingle with each other. By the end of stage 14, the epidermis already covers about 80% of the ventrodorsal perimeter. Intersegmental folds persist during epidermal spreading, so that segmental individuality is also maintained after closure.
The midgut has closed ventrally during the previous stage and then, in stage 14, dorsal closure of the midgut proceeds. At the end of stage 14, the midgut has a characteristic heart-like shape.
Head involution, initiated at the end of stage 13, continues. Head involution progresses simultaneously with the dorsalward extension of the epidermis to achieve dorsal closure. The hypopharyngeal lobes have been displaced into the stomodeum, and accordingly the salivary duct can now be seen ending in the floor of the atrium. The gnathal appendages have moved anteromedially; whereas the labial appendages of both sides join at the midline and move further cephalad to form the most anterior part of the mouth floor. Maxillary and mandibular appendages come to lie behind the lateral borders of the stomodeum and the lateral walls of the atrium, respectively.
Pharynx and oesophagus can be distinguished within the foregut. Also the globular proventriculus becomes clearly visible in the anterior midgut region. The hindgut grows considerably during stage 14, acquiring a hooked shape. It consists of a tube that opens in the anal pads and extends anteriorly up to 50% egg length (0% egg length = posterior pole). There it bends and courses further ventrocaudally to 30% egg length, to connect up with the midgut. The origin of the Malpighian tubules lies within this most anterior part of the hindgut, immediately posterior to the junction with the midgut and shortly before the bend. At this stage Malpighian tubules form four thin tubules. The anus is now surrounded by the epidermis of the anal plate.The somatic musculature, although already attached to the apodemes, is not yet completely stretched, nor can the normal larval pattern be recognized.
Cytodifferentiation, i.e. outgrowth of axonal processes, begins in sensory organs. The posterior spiracles become evident in the living embryo.
Stage 14, in vivo (0.8 MB)
Stage 14, sagittal sections (0.6 MB)..........additional information
Stage 14, virtual micrsocope, sagittal sections..........additional information
Stage 14, anti-Crumbs..........additional information