Page 79 - Anatomy and Physiology of Farm Animals, 8th Edition

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into a longitudinal rod, the notochord. The persists in the adult as the ventricular
system of the brain and the central canal of
notochord in vertebrates is essential to the
VetBooks.ir formation of the overlying nervous system the spinal cord (see Chapter 10).
and the differentiation of adjacent meso-
derm into definitive vertebrae.
Ectoderm superficial to the notochord Mesodermal Differentiation
is induced by it to thicken, forming the
neural plate. From this point, these cells, Mesodermal cells on each side of the
destined to become the nervous system, notochord condense into a series of block‐
constitute the neurectoderm. The lateral like, paired masses. These are the seg-
edges of the neural plate thicken and grow mentally arranged somites, which will
dorsad, turning the neural plate into a neu- give rise to vertebrae, dermis, and
ral groove. The growth of the edges of the muscles. Mesoderm lateral to the somites
neural groove continues until they meet is called nephrogenic (intermediate)
and fuse, forming the neural tube mesoderm. It will give rise to urogenital
(Fig. 3‐4), a longitudinally oriented tube on organs (Fig. 3‐5).
the dorsal midline of the body. The cells of As the flattened, disk‐like embryo
the neural tube will develop into the brain begins to fold into a more tubular shape,
and spinal cord. These steps (neurulation) the most lateral mesoderm, the lateral
create a stage of development called the plate mesoderm, splits into two layers,
neurula. The lumen of the neural tube forming a cavity, the coelom. The coelom

Ectoderm
Notochord

Mesoderm
Neural plate
Endoderm
Neural groove

Neural tube

Figure 3-4. Neurulation. Shown in cross‐section, the ectoderm overlying the notochord is induced to
thicken, forming the neural plate. Differential growth of cells of the neural plate produces a depression,
the neural groove. The edges of the neural groove approach one another and fuse, creating the neural
tube, the precursor to the spinal cord and brain.