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inside and become the nervous system, whereas the
AB progeny that remain on the surface spread out
to form the hypodermis, a syncytial covering of the
animal. Most of the neurons arise in this way; of the
222 neurons in the newly hatched C. elegans , 214 arise
from the AB lineage, whereas 6 are derived from the
MS blastomere and 2 from the C blastomere.
Cellular blastoderm
The development of the fruit fly, Drosophila , is char-
acteristic of the “long germ band” arthropods. Unlike
the embryos of the nematode and the leech, where
cleavage of the cells occurs at the same time as nuclear
divisions, the initial rounds of nuclear division in the
Drosophila embryo are not accompanied by correspon-
ding cell divisions. Instead, the nuclei remain in a syn-
cytium up until just prior to gastrulation, three hours
after fertilization. Prior to this time, the dividing nuclei
lie in the interior of the egg, but they then move out
toward the surface and a process known as cellular-
ization occurs, and the nuclei are surrounded by
plasma membranes. At this point the embryo is known
as a cellular blastoderm.
The major part of the nervous system of Drosophila
arises from cells in the ventrolateral part of the cellu-
lar blastoderm (Figure 1.6, top). Soon after cellulariza-
tion, the ventral furrow, which marks the beginning of
gastrulation, begins to form (Figure 1.6, middle). At the
ventral furrow, cells of the future mesoderm fold into
the interior of the embryo. The process of invagination
occurs over several hours, and the invaginating cells
will continue to divide and eventually will give rise to
the mesodermal tissues of the animal. As the meso-
dermal cells invaginate into the embryo, the neuro-
genic region moves from the ventrolateral position to
the most ventral region of the animal (Figure 1.6). The
closing of the ventral furrow creates the ventral
midline, a future site of neurogenesis. On either side
of the ventral midline is the neurogenic ectoderm,
tissue that will give rise to the ventral nerve cord, oth-
erwise known as the central nervous system (Figure
1.6). However, it is worth noting that a separate neu-
rogenic region, known as the procephalic neurogenic
region, gives rise to the cerebral ganglia or brain.
Drosophila neurogenesis then begins in the neuro-
genic region; they enlarge and begin to move from the
layer into the inside of the embryo (Figure 1.6). At the
beginning of neurogenesis, the neurogenic region is a
single cell layer; the first morphological sign of neuro-
genesis is that a number of cells within the epithelium
begin to increase in size. These larger cells then
undergo a shape change and squeeze out of the epithe-
lium. This process is called delamination and is shown
Neurogenic region
Ventral nerve cord
FIGURE 1.6 The nervous system of the Drosophila is derived
from the ventrolateral region of the ectoderm. The embryo is first
(top) shown at the blastoderm stage, just prior to gastrulation. The
region fated to give rise to the nervous system lies on the ventral-
lateral surface of the embryo (red). The involution of the mesoderm
at the ventral surface brings the neurogenic region closer to the
midline. Scattered cells within this region of the ectoderm then
enlarge, migrate into the interior of the embryo, and divide several
more times to make neurons and glia. These neurons and glia then
condense into the ganglia of the mature ventral nerve cord (or CNS)
in the larva and the adult.
in more detail in Figure 1.7. The cells that delaminate
are called neuroblasts and are the progenitors that will
generate the nervous system. In the next phase of
neurogenesis, each neuroblast divides to generate
many progeny, known as ganglion mother cells (GMCs).
Each GMC then generates a pair of neurons or glia. In
this way, the entire central nervous system of the larval
Drosophila is generated. However, the Drosophila
nervous system is not finished in the larva, but rather
additional neurogenesis occurs during metamorpho-
sis. Sensory organs, like the eyes, are generated from
imaginal discs, small cellular discs in the larva that
undergo a tremendous amount of proliferation during
metamorphosis to generate most of what we recognize
as an adult fly.
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