Healthcare and Medicine Reference
In-Depth Information
Protists
Egg
Colonial
flagellates
Vegetal
Radiata
Animal
Sponges
Bilateria
Ventral
Dorsal
Cnidarians
Vegetal
Deuterostomes
Protostomes
Nematodes
Platyhelminthes
Echinoderms
Molluscs
Annelids
Tunicates
Arthropods
Vertebrates
Blastula
FIGURE 1.1 Neurons throughout the evolution of multicellular
organisms have had many features in common. All animals other
than colonial flagellates and sponges have recognizable neurons that
are electrically excitable and have long processes. The Cnidarians
have nerve networks with electrical synapses, but synaptic trans-
mission between neurons is also very ancient.
Gastula
FIGURE 1.2 The early processes of animal development follow
a conserved pattern; after fertilization, a series of cleavage divisions
divide the egg into a multicellular blastula. The animal and vegetal
poles represent an initial asymetry in the oocyte, and the second axis,
dorsal-ventral in this example, is established after fertilization. The
process of gastrulation brings some of the cells from the surface of
the embryo to the inside and generates the three-layered structure
common to most multicellular animals.
gastrulation. Gastrulation can occur via a variety of
mechanisms, but all result in an inner, or endodermal,
layer of cells, an outer layer of cells, the ectoderm, and
a layer of cells between the two other layers, known as
the mesoderm (Gilbert and Raunio, 1997). The middle
layer can be derived from either the ectoderm (ectome-
soderm) or the inner layer (endomesoderm). During
the process of gastrulation, the cells of the mesoderm
and endoderm move into the inside of the embryo,
often at a single region, known as the blastopore. Once
the endoderm and mesoderm are inside the ball, they
usually obliterate the blastocoel and form a new cavity,
the archenteron, or primitive gut. Animals can be
divided in two on the basis of whether the mouth
forms near the point of this blastopore (in proto-
stomes) or at a distant site (in deuterostomes). Once
these three primary germ layers are established, the
development of the nervous system begins. A more
detailed description of the development of the other
organ systems is beyond the scope of this text. Never-
theless, one should keep in mind that the development
of the nervous system does not take place in a vacuum,
but is an integral and highly integrated part of the
development of the animal as a whole.
 
 
 
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