315In vertebrates, the phylotypic stage occurs around the pharyngula stage, just
before the limb buds appear, when the pharyngeal arches are evident, the heart is
beating, and 20 or more somites are present in the paraxial mesoderm (Fig. 7.3b).
There are very few features at this stage that can be used to distinguish a fish embryo,
from a frog or mammalian embryo. Some differences are only superficial, and mask
an underlying similarity. The extraembryonic yolk, which can vary greatly in size
from species to species, can superficially alter the appearance of the embryo. This
is misleading, however, because the yolk does not contribute any tissue to the body
(Richards 2009 ). Embryo size can also vary greatly from species to species at the
phylotypic stage, but the overall body plan is constant. Some differences at the phy-
lotypic stage, however, reveal important distinctions between the species. For
instance, marsupial embryos can be readily distinguished by the precocious devel-
opment of the forelimbs (Fig. 7.3) (Keyte et al. 2007 ). Marsupials spend a compara-
tively short time in the uterus in comparison with other mammals, and are born in a
relatively immature state (Smith 2001 ). Their forelimbs are fully formed and func-
tional, which enables the developing fetus to crawl out of the birth canal and into the
pouch unassisted by the mother, where it completes the developmental program
(Gemmell et al. 2002 ).
The conservation of morphology and gene expression at the phylotypic stage
suggests that there are unknown evolutionary constraints on the morphology of the
body plan that do not exist at early or late stages. Alternatively, the conservation of
morphology may simply be due to the relative lack of evolutionary pressure driving
morphological changes at this stage in comparison with the adult stages.
7.3 Blastoderm Formation and Gastrulation
7.3.1 Amniotes
7.3.1.1 Birds
Vertebrate embryos display enormous morphological diversity at the blastoderm
stage, consistent with the Developmental Hourglass model. Avian and reptilian
embryos are specialized for developing on land, outside the mother. They develop
inside a shell that protects against evaporation while promoting gas exchange, and
contain a large yolk that acts as a source of nutrition until hatching. The early cell
divisions undergo meroblastic cleavages, which are characterized by incomplete
cytokinesis that does not partition the yolk into the blastomeres, called. The earliest
stages of embryonic development of birds occur in utero. Upon egg laying, the
chicken blastoderm appears as a tiny flat disc composed of about 20,000 cells on top
of the yolk, which expands to about 55,000 cells before gastrulation (Fig. 7.3a). The
blastodisc is organized as a translucent circle of cells, called the area pellucida (ap)
surrounded by an outer area, called the area opaca (ao) because it is not translucent.
The outer ring of the area pellucida is called the Marginal Zone, and consists of cells
that primarily contribute to extraembryonic tissues.
7 Establishment of the Vertebrate Germ Layers