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FIGURE 1.10 Development of the zebrafish embryo. The zebrafish embryo develops primarily on top of
a large ball of yolk. The cleavages are confined to the dorsal side. After multiple cleavage divisions, the cells
migrate over the yolk in a process called epiboly. Once the cells have enclosed the yolk, the development of
the nervous system proceeds much like that described for the frog. (After Kimmel et al., 1995)
dermal cells migrate toward the animal pole on the
shield side, the ectoderm above becomes committed to
a neural fate and a definitive neural plate begins to
form. One of the best things about the zebrafish
embryos, as far as developmental biologists are con-
cerned, is their extraordinary optical transparency.
Avian embryos are an extreme example of a “yolky”
egg. We all know how much yolk is in a chicken egg.
As a result, when the single large egg cell begins to
divide, the cell cleavage divisions do not penetrate into
this yolk but are restricted to the relatively yolk-free
cytoplasm at the animal pole. These cleavages lead to
a disc of cells, called the blastodisc , which is essentially
floating on the yolk. The invagination of mesoderm
occurs in this disc through a blastopore-like structure
known as the “primitive streak.” During this invagi-
nation, future mesoderm cells migrate into the interior
of the embryo (Figure 1.11).
What about mammalian embryos, which have
essentially no yolk and derive all their nourishment
from the placenta? The cleavage divisions of mam-
malian embryos are complete (Figure 1.12), and the
resulting cells are equal in their potential; there
is no obvious animal or vegetal pole. However, after
a sufficient number of divisions, when the blastula
forms, there are cells on the inside of the ball, called
the inner cell mass , that produce the embryo, whereas
the cells on the outside of the ball make the placenta
and associated extra-embryonic membranes. Even
though they lack yolk, mammalian embryos undergo
a process of gastrulation that is similar to the avian
embryo in that the developing mesodermal cells
migrate through the primitive streak to reach the inte-
rior (Figure 1.12). The primitive streak runs along the
anterior-posterior axis of the embryo, and the ecto-
derm laying above the ingressing mesodermal cells
becomes the neural plate and subsequently the neural
tube, much like that described above for the other ver-
tebrate embryos.
The three basic layers of the embryo—the endo-
derm, mesoderm, and ectoderm—arise through the
complex movements of gastrulation. These move-
ments also create new tissue relations. For example,
after gastrulation in the frog, presumptive mesoderm
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