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The Stages of Embryogenesis page 2 | 3 | 4
Stage 5: Embryogenesis starts with cleavage (stages 1- 4) during which the nucleus of the fertilized egg performs 13 rapid divisions. Most of the resulting nuclei become arranged in a single layer beneath the egg surface, and cell membranes are formed around the nuclei. This leads to the cellular blastoderm (stage 5), a homogeneous cellular sheet surrounding the central yolk. The germ-line cells, also called pole cells (pc), stand out as a cluster of 34-37 round cells at the posterior embryonic pole.
Stage 8: During gastrulation (stages 6-8), cells within the polar caps and the mid-ventral part of the blastoderm invaginate. The three germ Layers are generated by this movement.
Most of the cells that remain at the surface represent the ectoderm; the invaginating cells form the endoderm (anterior and posterior midgut rudiments) and the mesoderm. A narrow mid-dorsal partition of the blastoderm gives rise to the amnioserosa (as), a thin membrane that covers the germ band dorsally. Coincident with gastrulation is the beginning of germ-band elongation, a movement that pushes the posterior tip of the germ band upward and then toward anterior. Several transitory furrows are formed during germ-band elongation. The deepest one of these furrows (cephalic furrow, cf) completely surrounds the embryo. Anterior to the cephalic furrow lies the procephalon (pro); posterior to the cephalic furrow lies the segmented germ band (gb). At the posterior tip of the germ band lies the amnioproctodeal invagination (pr), a pocket formed by the invaginating posterior midgut rudiment (endoderm) and the surrounding ectoderm that becomes the hindgut.
Stage 9: Following gastrulation, the germ band elongates further (stages 9-11). The ectoderm begins to split up into numerous different organ primordia (foregut and hindgut, CNS, epidermis). The cephalic furrow is still present; the other transient furrows have all but disappeared.