319forms the blastocoel in the middle of the embryo (Manejwala et al. 1989 ). The cells
remaining inside the embryo after compaction form the Inner Cell Mass (ICM), a plu-
ripotent group of cells that have the ability to generate any tissue in the embryo and
some extraembryonic tissues (Beddington and Robertson 1989 ). The first cells from
the ICM to differentiate form the visceral endoderm (VE), an extraembryonic tissue
that separates the ICM from the blastocoel (Gardner 1983 ). The embryo at this stage is
called a blastocyst (Fig. 7.3a). In most placental mammals, the epiblast forms a flat disc
after implantation, similar to the disc observed in avian/reptilian blastoderms. Some
rodents, however, like mice and rats, form an epiblast shaped like an inverted cup (Fig.
7.3a). In one strain of mice, the epiblast consists of only 660 cells just prior to the onset
of gastrulation, but this expands rapidly to 14,500 cells by mid- gastrulation, due to a
dramatic shortening of the cell cycle (Snow 1977 ). In the mouse, the primitive streak
forms along the posterior side of the inverted cup, with endoderm and mesoderm
egressing from the epiblast to surround the future ectoderm. A thickened tissue, called
the node, marks the anterior of the streak.
7.3.1.5 Mammals—Metaeutherians
Marsupials are classified as non-placental mammals, or metatheria, because they
undergo a significant portion of their development in a maternal pouch. Despite this
categorization, marsupial embryos attach to the uterus and produce a yolk sac placenta,
which has the same embryological origin as the yolk sac in eutherian mammals (Freyer
et al. 2003 ). Marsupials do have active vitellogenin genes, and produce a yolk mass
visible at the blastocyst stage (Fig. 7.3a) (Selwood et al. 1997 ). Marsupial blastocysts
are also distinct because they do not form an ICM. Instead, they initially take on a uni-
laminar structure and resemble a hollow ball (Fig. 7.3a). This structure arises because
the blastomeres strongly adhere to the zona pellucida during the early cleavages
(Selwood 1986 , 1992 ). Nonetheless, cells are allocated at this stage either to an extra-
embryonic trophoblast lineage or a pluripotent pluriblast lineage that generates the
entire epiblast (Cruz et al. 1996 ). These cell populations can be distinguished based on
morphological criteria, since the pluriblasts are large, round cells and the trophoblast
cells are smaller and flattened (Selwood and Johnson 2006 ). The pluriblasts divide to
generate a bilaminar blastocyst consisting of an extraembryonic hypoblast, or parietal
endoderm, and an epiblast/pluriblast (Mate et al. 1994 ; Selwood et al. 1997 ; Kress and
Selwood 2006 ). Growth of the embryo generates an a flat, pear-shaped epiblast visible
on top of the trophoblast by the time of gastrulation (Cruz et al. 1996 ).
7.3.2 Amphibians
Amphibians are a large family of tetrapods that includes frogs, salamanders, and
newts. They evolved to live on land as adults, but their embryonic and larval life
occurs in an aquatic environment. The eggs are large and full of yolk platelets that
become internalized into the blastomeres after fertilization, with the heavier
7 Establishment of the Vertebrate Germ Layers