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THE DORSAL, HOLLOW NERVE CORD
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THE FORMATION OF THE NERVOUS SYSTEM The primitive streak of the epiblast marks the point where the central nervous system
begins to form. A neural plate forms two neural folds which will fuse
to form the neural tube. |
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Although the organizing region of amphibians
forms a circular blastopore compared to the linear
primitive streak of amniotes, the two regions are similar. In amniotes, Henson’s node at the anterior end
of the primitive streak is homologous to the dorsal blastopore
lip of amphibians, as evidenced by shared gene expression such as goosecoid, Xnot/Cnot, noggin, and
nodal/XNr in addition to physical and functional
similarities (Arendt, 1999). |
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Once the primitive streak forms, if it possible
to identify the cranial/caudal and dorsal/ventral axes of the embryo.
By the fourth week the primitive streak has disappeared although
it does occasionally persist later in development in some fetuses.
In 1/35,000 children the tissue which persists from the primitive
streak forms a saccrococcygeal teratoma ( The nervous system forms as neurectoderm differentiates to neural tissue. The neural plate forms two neural folds which,
upon fusing, become the neural tube. Frog Neural Plate |
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Frog
Neural Folds |
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Frog
Neural Tube |
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HOLLOW NERVE CORD AND SPINAL CORD One of the four basic characteristics of
chordates is the presence of a hollow, dorsal nerve tube. In adults, nervous tissue surrounds the central
canal of the spinal cord and the ventricles of the brain through which
cerebrospinal fluid passes. The
small relative size of these spaces in adult nervous systems is unlike
the large hollow center of the embryonic nerve cord. Frog |
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chick brain | |||||||||||||||||||||
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pig brain | |||||||||||||||||||||
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Pig
spinal cord |
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20
mm pig brain showing lateral ventricles and fourth ventricle |
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Chimps
and humans have the shortest spinal cords (relatively) in the order primates
(MacLarnon, 1996).
Human embryos begin with coccygeal ganglia which degenerate although a coccygeal medullary vestige is retained
until later in fetal life (Ariens, p. 221).
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