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different vertebrates: amphibians, fish, birds, and
mammals. In all of these animals, multiple cleavage
divisions generate a large number of cells from the
fertilized oocyte. However, while gastrulation in all of
these animals is basically conserved, the details of the
cellular movements during this phase can look quite
Amphibian eggs are like those of many animals in
that the egg has a distinct polarity with a nutrient-rich
yolk concentrated at the “vegetal” hemisphere and a
relatively yolk-free “animal” hemisphere. After fertil-
ization, a series of rapid cell divisions, known as cleav-
ages, divides the fertilized egg into blastomeres. The
cleavage divisions proceed less rapidly through the
vegetal hemisphere, and by the time the embryo
reaches 128 cells, the cells in the animal half are much
smaller than those of the vegetal half (Figure 1.8). The
embryo is called a blastula at this stage. The process by
which the relatively simple blastula is transformed
into the more complex, three-layered organization
shared by most animals, is called gastrulation. During
this phase of development, cells on the surface of the
embryo move actively into the center of the blastula.
The point of initiation of gastrulation is identified on
the embryo as a small invagination of the otherwise
smooth surface of the blastula, and this is called the
blastopore (Figure 1.8). In amphibians the first cells to
invaginate occur at the dorsal side of the blastopore
(Figure 1.8), opposite to the point of sperm entry. As
described below, these cells have a special significance
to the development of the nervous system. The mech-
anism of involution is complex, and it appears that
a small group of “bottle” cells initiate the process
by changing shape and creating a discontinuity in the
The involuting cells lead a large number of cells that
were originally on the surface of the embryo to the inte-
rior (Figure 1.8). The part of the blastula that will ulti-
mately reside in the interior of the embryo is called the
involuting marginal zone (IMZ). Most of these cells will
give rise to mesodermally derived tissues, like muscle
and bone. The first cells to involute crawl the farthest
and produce the mesoderm of the anterior part of the
animal (i.e., the head). The later involuting IMZ cells
produce the mesoderm of more posterior regions,
including the tail of the tadpole. At this point in devel-
opment, the neural plate of the vertebrate embryo still
largely resembles the rest of the surface ectoderm.
However, shortly after its formation, the neural plate
begins to fold onto itself to form a tubelike structure, the
neural tube (Figure 1.9). Much more will be said about
the neural tube and its derivatives and shape changes in
the next two chapters. For now, suffice it to say that this
tube of cells gives rise to nearly all the neurons and glia
Neurogenic region
FIGURE 1.7 The neuroblasts of the Drosophila separate from the
ectoderm by a process known as delamination. The neuroblasts
enlarge relative to the surrounding cells and squeeze out of the
epithelium. The process occurs in several waves; after the first set of
neuroblasts has delaminated from the ectoderm, a second set of cells
in the ectoderm begins to enlarge and also delaminates. The delami-
nating neuroblasts then go on to generate several neurons through
a stereotypic pattern of asymmetric cell divisions. The first cell divi-
sion of the neuroblast produces a daughter cell known as the gan-
glion mother cell, or GMC. The first ganglion mother cell divides
to form neurons, while the neuroblast is dividing again to make
another GMC. In the figure, the same neuroblast is shown through
its successive stages as Nb, while the GMCs are numbered succes-
sively as they arise.
Vertebrate embryos undergo a fundamentally
similar process of early development, though at first
appearance they seem to be quite different. In this
section we will review the development of several
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