Biology Reference
In-Depth Information
Figure 4.5
Early embryonic stages of
Drosophila melanogaster
from fertilization to just before gastru-
lation, showing the appearance of pole and somatic buds and cessation of division of yolk nuclei.
Numbers indicate division cycles; each cycle begins with the start of interphase and ends at the con-
clusion of mitosis. Embryos are in longitudinal section without the vitelline membrane. All nuclei
(black circles) are shown for cycles 1-5 and afterwards only some are shown. Stippled areas rep-
resent yolk and open areas represent yolk-free cytoplasm. Yolk-free cytoplasm is found both at the
periphery (
=
periplasm) and in islands around the nuclei.
During Cycles 1-7, nuclei multiply exponentially in the central region of the fertilized egg. Cycle 8 illus-
trates migration of the majority of the nuclei to the periphery, leaving the future yolk nuclei behind in
the center. Yolk nuclei continue to divide in synchrony with other nuclei in cycles 8-10; they then cease
dividing and become polyploid. Early in cycle 9, a few nuclei appear in the posterior periplasm and
cause protrusions of the cytoplasm, called pole buds. During cycle 10, the remaining migrating nuclei
enter the periplasmic region, forming somatic buds over the entire embryonic surface. During the
tenth cycle, pole buds are pinched off to form pole cells. After this, synchrony between the pole cells
and the syncytium is lost. The syncytial nuclei continue to divide synchronously. The periplasm begins
to thicken in cycle 13. During cycle 14, the formation of a plasma membrane begins to separate cells
over the entire surface of the embryo, with nuclei elongating to match elongated cells formed by late
cycle 14A. During 14B, gastrulation movements begin with the infolding of the cephalic furrow (ante-
rior) and posterior midgut furrow, and subsequently the cells no longer divide synchronously.
