323sion of axial mesoderm from the archenteron roof (Bolker 1993a). Secondly, the blas-
topore forms at the equator in sturgeon, instead of subequatorially as in amphibians.
The embryo then undergoes a dramatic thinning of the marginal zone and blastocoel
roof, likely by radial intercalation (Bolker 1993b). This thinning process resembles the
epiboly movements in teleost embryos, and acts to push the bottle cells into a more
vegetal location. The process of involution is very similar in bichir and lamprey,
although gastrulation in the lamprey has not been described in extensive detail (Piavis
1961 ; Takeuchi et al. 2009a). Bottle cells have not been described in the lamprey,
but the presumptive mesoderm and endodermal appear to internalize by a process of
involution similar to that in amphibians. There is some controversy about the origins of
the definitive endoderm in these species (Richardson et al. 2010 ).
7.3.5 Conclusion
The comparison of blastoderm structure and gastrulation in different vertebrate
phyla is instructive to understand how gastrulation evolved. The origin of the verte-
brate lineage coincided with the acquisition of a large yolky mass, which influenced
the mode of early cleavages and gastrulation (Arendt and Nubler-Jung 1999 ). The
basal vertebrates lamprey and bichir (Polypterus) have large extraembryonic yolk
cells and undergo holoblastic cleavage. The presence of yolk in the vegetal pole of
lamprey and Polypterus places constraints on cell movements that influence the
mechanics of gastrulation. Yolk cells also influence the timing and mechanism of
gastrulation in vertebrates with internalized yolk. In Xenopus, for instance, the pres-
ence of the yolky endodermal cells in the vegetal pole directs the constriction of the
bottle cells toward the equator, which in turn controls the involution movements of
mesoderm during gastrulation (Hardin and Keller 1988 ). The subequatorial location
of the bottle cells is important for Xenopus morphogenesis, and a more equatorial
position was predicted to cause a high frequency of exogastrulation. Consistent with
this prediction, in the bottle cells in sturgeon embryos are formed at the equator, but
gastrulation is delayed until radial intercalation movements extend the embryo and
bring the bottle cells to a more vegetal location (Bolker 1993a).
Embryos with a larger extraembryonic yolk mass, like teleosts, reptiles, and
birds undergo meroblastic cleavage (Sheng 2015 ). If the yolk is large enough, the
blastoderm forms a disc instead of a sphere. The reptile blastoderm is a flat disc on
a large yolk. These embryos undergo meroblastic cleavage, but form a blastopore
and involute rather than forming a primitive streak and node. Thus gastrulation in
reptiles occurs by an intermediate mechanism, between that of amphibians and of
other amniotes. Monotremes are a family that includes egg-laying mammals like the
platypus and echidnas. They occupy a similarly intermediate position in the verte-
brate phylogeny and their embryos follow a bird-like pattern of meroblastic cleav-
ages followed by formation of a node and primitive streak (Wilson and Hill 1915 ;
Hughes and Hall 1998 ; Werneburg and Sánchez-Villagra 2011 ). As mammals lost
their yolk genes, they retained their disc-like morphology, and elaborated extraem-
bryonic structures to facilitate survival in the womb.
7 Establishment of the Vertebrate Germ Layers