Blastoderm cell formation takes place, while blastoderm nuclei elongate considerably (2:10-2:50 h). At the end of stage 5, pole cells begin to shift their position dorsalwards. Shifting of pole cells can be observed directly. At the same time, midventral blastoderm cells acquire an irregular, wavy appearance, darkening at their interface with the yolk. This precedes their invagination to form the mesodermal and the anterior endodermal primordia.
Cellularization occurs by means of the introgression of membrane furrows to separate single blastoderm nuclei. This is a rapid process, and is accomplished within 30 min at 25°C. Blastoderm nuclei are spherical at the onset of cellularization but elongate considerably as the process continues, increasing in length from 3-4 µm to 10-15 µm.
Blastoderm cells around the perimeter of the entire egg at this stage are not completely isolated since they still maintain connected with the syncytial yolk cytoplasm through wide cytoplasmic bridges. These connections are lost during gastrulation.
All somatic blastoderm nuclei and cells have the same shape and do not show any apparent differences between particular egg regions. Both shape and size, however, will show considerable regional variations during the next stage.
Stage 5, sections..........additional information
Cellularization, in vivo (0.4 MB)
Cellularization, schematic (1 MB)
Stage 5, interactive (3.2 MB)