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Vertebrate Morphogenesis

Introduction
 Life history begins with fertilization, followed by embryonic

development (ontogeny), maturation, and in some cases
senescence (aging)
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 Structures of two species that pass through closely similar
steps of embryonic development can be taken as evidence of
homology between these structures
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Homology
 Evolution is a process of descent with modification:

Characteristics present in an ancestral organism are altered
(by natural selection) in its descendants over
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 Similarity resulting from common ancestry is known as
homology
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Rather, the underlying skeletons of the arms,
forelegs, flippers, and wings of different mammals are homologous structures
that represent variations on a structural theme that was present in their common
ancestor
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 If species share features because of convergent evolution, the
resemblance is said to be analogous
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Although they evolved independently from different
ancestors, these two mammals have adapted to similar
environments in similar ways
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 During these early stages, the embryonic area becomes
delineated from the extraembryonic area that supports
the embryo or delivers nutrients
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The Chordate Egg
 Although an egg can be very large, as is a chicken egg, it is but a

single cell with a nucleus, cytoplasm, and cell membrane, or plasma
membrane
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Once in the ovum, vitellogenin is transformed into
yolk platelets consisting of storage packets of nutrients that help
support the growing needs of the developing embryo
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 Further,

the yolk can be evenly distributed
(isolecithal) or concentrated at one pole
(telolecithal)
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Parts of the Chordate ovum
 The region beneath the plasma membrane of the ovum is

referred to as the cortex
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 Outside the plasma membrane, three envelopes surround the
ovum
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Consists of
a layer called vitelline membrane, a transparent jacket of
fibrous protein
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 The secondary egg envelope is composed of ovarian, or

follicle, cells that immediately surround and help transfer
nutrients to the ovum
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 The successful sperm must penetrate all three layers—
follicle cells (in eutherian mammals), vitelline membrane,
and plasma membrane
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In some sharks, it consists of an egg case
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The tertiary layer is added after fertilization when the ovum
travels down the uterine tubes
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Lecithotrophic nutrition occurs through direct
transfer of yolk to the connecting part of the digestive tract or
through the vitelline arteries and veins
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Vascular placentae or secretions of the oviduct that deliver
nutrients to embryos are examples
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 Activation of development, initiated by sperm penetration,

ushers in the next process, cleavage
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 The zygote is transformed from a single cell into a solid mass
of cells called the morula
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 The first cleavage furrows appear at the animal pole and

progress toward the vegetal pole
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 In embryos where yolk is plentiful, cell division is impeded,
mitotic furrowing is slowed, only a portion of the cytoplasm
is cleaved, and cleavage is said to be meroblastic
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