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2 cell stage
4 cell stage
Blastula
Uterine
wall
Inner
cell mass
Blastocyst
Implantation
Neural fold
Somites
Primitive pit
Primitive streak
Primitive streak
Embryo
gastrulation
Blastocyst
FIGURE 1.12 Development of the human embryo. The initial cleavage divisions are symmetric and
produce apparently identical blastomeres. There is not much yolk in the mammalian embryo, since most
nutrients are derived from the placenta. After multiple cleavage divisions, the embryo is called a blastocyst
and develops a distinct inner cell mass and an outer layer of cells. The inner cell mass will develop into the
embryo, while the outer cells will contribute to the placenta. After implantation, the embryo begins to elon-
gate, and a section through the amniotic cavity shows that the human embryo develops a primitive streak
much like that present in the chick embryo. The primitive streak is a line of cells migrating into the blasto-
coel that will form the mesoderm, and the neural tube will form from the ectoderm overlying the involuting
mesoderm. The tube rolls up and forms the brain and spinal cord in a process much like that described for
the other vertebrates.
embryo, where neural tissue does not normally arise.
To determine whether the new neural tissue that
developed in these twinned embryos came from the
dorsal lip tissue, they transplanted the dorsal blasto-
pore lip from the embryo of a normal pigmented frog
into the embryo of a nonpigmented strain of frogs.
They found that the second body axis that resulted was
made of mostly nonpigmented cells, indicating that it
came largely from the host blastula, not the trans-
planted dorsal lip. Thus, the grafted blastopore cells
have the capacity to induce neural tissues from a
region of the ectoderm that would normally not give
rise to a nervous system. In addition to the neural
tissue in these embryos, they found that mesodermally
derived structures also contributed to the twinned
embryo. They concluded that the dorsal lip acts not
only as a neural inducer but also as an “organizer” of
the entire body axis. As a result of these experiments,
this region of the embryo is known as the Spemann
organizer.
In the years following these initial studies of
Spemann and Mangold, several embryologists tried to
further characterize the induction process, as well as
to identify the inducing principle or factor. One of the
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