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each other at the phylotypic stage, but there is considerable morphological diversity
at earlier and later stages. Vertebrate embryos fall into one of two broad categories
at the blastoderm stage, depending largely upon the amount of yolk contained in the
eggs. Embryos with internal yolk platelets, like amphibians, or a relatively small-
sized extraembryonic yolk cell, like the teleosts, have a spherical blastoderm.
Embryos with a large extraembryonic yolk cell, like birds, reptiles, and mono-
tremes, have a disc shaped blastoderm. In most eutherian mammals, the epiblast is
disc-shaped despite the loss of yolk, suggesting that other morphological constraints
act in the uterus to maintain this morphology. The exception to this is the rodent
family, which has an inverted cup shaped epiblast.
Vertebrate embryos employ a variety of different mechanisms to internalize the
presumptive mesoderm and endoderm. For example, the mesoderm and endoderm
are internalized primarily by involution in basal vertebrates, sturgeon and amphib-
ian embryos, and by ingression in birds and mammals. In reptiles, mesoderm and
endoderm are internalized by involution through a blastopore, despite their disc-
shape. Ingression occurs in a subset of mesodermal populations in some amphibians
and reptiles. Thus, reptilian embryos seem to represent an intermediate state
between the amphibian and avian forms of gastrulation. In teleosts, mesoderm and
endoderm internalize by synchronous ingression, which differs from involution
because the cells in a teleost blastoderm are not tightly connected to each other, as
they are in an amphibian embryo. These different mechanisms may reflect the
amount of cell movement in the pre-gastrula stages. Amphibians have relatively
little cell mixing before gastrulation, whereas teleosts, birds, and mammals have a
high degree of cell mixing. It is not clear how much cell mixing there is in the reptile
blastodisc, or in the blastula of basal vertebrates, because no high-resolution fate
maps have been produced for these early stages.
Despite these different mechanisms of internalization, the molecular mecha-
nisms underlying germ layer induction are quite similar across the vertebrate phy-
lum. Signals of the Nodal subclass of the TGF-β superfamily are required to induce
and pattern the mesoderm and endoderm. These signals act in parallel with signals
of the Fgf family, typically Fgf4 and Fgf8 to induce mesoderm. Despite some prog-
ress in Xenopus, we know little about how cells integrate their response to these two
signaling pathways, or about how Fgf signals can act both to instruct cell fate and to
direct cell movements. The allocation of cells to the three germ layers is only the
beginning of the process of germ layer formation. We have not discussed the events
downstream of Nodal and/or Fgf signaling that result in the expression of mesoderm
or endodermal fate in vertebrates. There has been significant progress in elucidating
the events immediately downstream of Nodal signals in frogs and fish, but further
work is necessary to fully understand how mesoderm is distinguished from endo-
derm (Kimelman 2006 ; Kiecker et al. 2016 ). The events upstream of Nodal signal-
ing are also an area of future investigation. In frogs, expression of the xnr genes is
induced by the T-box transcription factor, VegT and maternally provided Activin,
but the upstream factors in other vertebrates are not known. It is likely a T-box pro-
tein is required in zebrafish, acting in parallel to maternally provided Ndr1/Sqt sig-
nals to induce zygotic expression of ndr1/sqt and ndr2/cyc, but this factor has not
W. Tseng et al.