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Despite the obvious morphological similarity to avian embryos, reptilian
embryos undergo a different method of gastrulation not observed in other amniotes.
Instead of forming a Node and primitive streak, cells converge in the posterior epi-
blast and form an amphibian-like blastopore (see below) (Fig. 7.4b). Cells in the
blastopore form a continuous rolling sheet of involuting cells, as in frogs, but then
they lose their epithelial polarity and ingress to form mesoderm and definitive endo-
derm (Fig. 7.4b) (Coolen et al. 2008 ; Bertocchini et al. 2013 ; Stower et al. 2015 ).
Prior to formation of the blastopore, cells in the PMZ flatten to form a structure
called the primitive plate, which is found in all reptiles (Fig. 7.4b) (Coolen et al.
2008 ; Bertocchini et al. 2013 ; Stower et al. 2015 ). Cells in the primitive plate inter-
nalize by ingression, and contribute to the definitive endoderm and an uncharacter-
ized type of mesoderm (Bertocchini et al. 2013 ). Thus, the mechanism that forms
the germ layers in reptiles is a hybrid between the involution based mechanism of
internalization found in amphibians, and amniote-like ingression.
7.3.1.3 Mammals—Monotremes
Mammalian embryos display a surprising amount of morphological diversity at the
blastula stage. Monotremes are a family of egg-laying mammals that includes the
platypus and the echidnas, making their embryonic development of particular evolu-
tionary interest. Embryological studies of this group have been limited because they are
protected species and difficult to breed in captivity. Platypus eggs have a large yolk and
a disc-shaped blastoderm, like those in birds and reptiles, complete with an extraem-
bryonic hypoblast (Wilson and Hill 1902 ; Selwood 1994 ; Hughes and Hall 1998 ;
Selwood and Johnson 2006 ). The blastoderm forms by meroblastic cleavages, like
those of reptiles and birds, and gastrulation occurs through a primitive streak located in
the posterior of the blastodisc (Wilson and Hill 1902 ; Hughes and Hall 1998 ).
7.3.1.4 Mammals—Eutherians
Placental mammals, or eutherians, are a group that includes the animals we are most
intimately familiar with, including the most commonly used mammalian developmen-
tal model organism, the mouse. The embryos in this group develop inside the mother and
produce an extraembryonic yolk sac, but they lack yolk due to an accumulation of mis-
sense mutations in the vitellogenin genes (Brawand et al. 2008 ). Holoblastic cleavages
divide the embryos into complete blastomeres. The early cleavages occur inside a
membrane called the zona pellucida and produce a small spherical structure at the
8-cell stage, which is called the morula stage because it looks like a mulberry. After the
8- or 16-cell stages, the embryo undergoes compaction, a change in morphology driven
by increased adhesion of cells on the outer surface of the embryo to each other. This
change is accompanied by a lineage restriction such that the outer cells generate the
trophoblast, an extraembryonic tissue specialized for invading the uterine wall and pro-
ducing the elaborate tissues required for survival inside the womb (Pierce et al. 1988 ).
The trophoblast express Na+, K+-ATPase pumps, which drive the influx of fluid that
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