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A
B
C Adult brain
Cephalic flexure
Cephalic flexure
Pontine flexure
Midbrain
Cervical flexure
Cerebellum
Cervical flexure
Medulla
Telencephalon
Telencephalon
Spinal cord
Lateral ventricle
Forebrain
Diencephalon
Forebrain
Midbrain
Hindbrain
Neural retina
3rd ventricle
Lens
Cerebral aqueduct
Mesencephalon
Metencephalon
(pons and cerebellum)
Midbrain
4th ventricle
Spinal
column
Hindbrain
Myelencephalon
(medulla)
Spinal cord
FIGURE 2.1 The vertebrate brain and spinal cord develop from the neural tube. Shown here as lateral
views (upper) and dorsal views (lower) of human embryos at successively older stages of embryonic devel-
opment (A,B,C). The primary three divisions of the brain (A) occur as three brain vesicles or swellings of the
neural tube, known as the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhomben-
cephalon). The next stage of brain development (B) results in further subdivisions, with the forebrain vesicle
becoming subdivided into the paired telencephalic vesicles and the diencephalon, and the rhombencephalon
becoming subdivided into the metencephalon and the myelencephalon. These basic brain divisions can be
related to the overall anatomical organization of the mature brain (C).
THE ANTERIOR-POSTERIOR
AXIS AND HOX GENES
Optic lobes
A
Protocerebrum
Antennal nerves
In both vertebrates and invertebrates, the mecha-
nisms that control the regional development of the
nervous system are dependent on the mechanisms
that initially set up the anterior-posterior axis of the
embryo. Much more is known about these mecha-
nisms in the Drosophila embryo, and so this will be
described first; however, it appears that many of the
same genes are involved in the specification of the
anterior-posterior axis in the vertebrate.
The anterior-posterior axis of the fly is primarily set
up by the distribution of two molecules: a transcrip-
tion factor known as bicoid , localized in the anterior
pole of the embryo, and a gene that codes for an RNA-
binding protein called Nanos , localized primarily in the
posterior pole of the embryo (Driever and Nusslein-
Volhard, 1988). The mRNAs for these genes are local-
ized in their distribution in the egg prior to fertilization
by the nurse cells in the mother. Shortly after fertiliza-
tion, these mRNAs are translated, resulting in oppos-
Deutocerebrum
Tritocerebrum
Subesophageal
ganglion
Segmental
nerves
Leg
B
b1 Protocerebrum
b2 Deutocerebrum
b3 Tritocerebrum
Segmental ganglia
FIGURE 2.2 The brain of the Drosophila develops from extensive
neuroblast delamination in the head. A. Three basic divisions of the
brain are known as the protocerebrum, the deutocerebrum, and the
tritocerebrum. B. These divisions are similar to the segmental ganglia
in that they are derived independently from delaminating neuro-
blasts in their respective head segments. However, they later fuse
together and along with the optic lobes form a complex network.
 
 
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