The Questions of Developmental Biology

and material can diffuse throughout the embryo. In these embryos, the specification of cell types
along anterior-posterior and dorsal-ventral axes is accomplished by the interactions of
cytoplasmic materials within the single, multinucleated cell. Moreover, the initiation of the
anterior-posterior and dorsal-ventral differences is controlled by the position of the egg within the
mother's ovary. Whereas the sperm entry site may fix the axes in ascidians and nematodes, the
fly's anterior-posterior and dorsal-ventral axes are specified by interactions between the egg and
its surrounding follicle cells.
Cleavage
Most insect eggs undergo superficial cleavage, wherein a large mass of centrally located
yolk confines cleavage to the cytoplasmic rim of the egg. One of the fascinating features of this
cleavage type is that cells do not form until after the nuclei have divided. Cleavage in a
Drosophila egg is shown in Figure 9.1. The zygote nucleus undergoes several mitotic divisions
within the central portion of the egg. In Drosophila, 256 nuclei are produced by a series of eight
nuclear divisions averaging 8 minutes each. The nuclei then migrate to the periphery of the egg,
where the mitoses continue,
albeit at a progressively slower
rate. During the ninth division
cycle, about five nuclei reach the
surface of the posterior pole of
the embryo.These nuclei become
enclosed by cell membranes and
generate the pole cells that give
rise to the gametes of the adult.
Most of the other nuclei arrive at
the periphery of the embryo at
cycle 10 and then undergo four
more divisions at progressively
slower rates.
During these stages of nuclear division, the embryo is called a syncytial blastoderm,
meaning that all the cleavage nuclei are contained within a common cytoplasm. No cell
membranes exist other than that of the egg itself.
Although the nuclei divide within a common cytoplasm, this does not mean that the
cytoplasm is itself uniform. Karr and Alberts (1986) have shown that each nucleus within the
syncytial blastoderm is contained within its own little territory of cytoskeletal proteins. When the
nuclei reach the periphery of the egg during the tenth cleavage cycle, each nucleus becomes
surrounded by microtubules and microfilaments. The nuclei and their associated cytoplasmic
islands are called energids. Figure 9.2 shows the nuclei and their essential microfilament and
microtubule domains in prophase of the twelfth mitotic division.