C h a p t e r 3 4
Organization and Control of Neural Function
831
CSF-filled cavities, the first and second (lateral) ven-
tricles. The remaining diencephalic portion of the neu-
ral tube develops into the thalamus and hypothalamus.
The neurohypophysis (posterior pituitary) grows as a
midline ventral outgrowth at the junctions of segments
1 and 2. A dorsal outgrowth, the pineal body, develops
between segments 2 and 3.
Segmental Organization
Developmentally, the basic organizational pattern of the
nervous system is that of a longitudinal series of segments,
each repeating the same fundamental pattern that is
retained in postnatal life. The CNS and its associated
peripheral nerves consist of approximately 43 segments,
ANTERIOR
POSTERIOR
Dorsal view
Rostral
neuropore
Somites
(mesoderm)
Caudal
neuropore
Neural plate
Neural folds
Neural groove
Cut edge
of amnion
Neural tube
Neural tube
Yolk sac wall
Neural groove
Neural folds
Neural folds
Neural plate
POSTERIOR
Dorsal view
Neural plate
Somites
Ectoderm
Ectoderm
Future
neural crest
Notochord
Mesoderm
Endoderm
Notochord
Mesoderm
Endoderm
Neural groove
Ectoderm
Notochord
Mesoderm Endoderm
A
B
Transverse sections
Neural crest
Neural
crest
1
2
3
1
2
3
FIGURE 34-5.
Folding of the neural tube.
(A)
Dorsal view of a six-somite embryo (22 to 23 days)
showing the neural folds, neural groove, and fused neural tube.The anterior neuropore closes at
about day 25 and the posterior neuropore at about day 27.
(B)
Three cross-sections taken at the levels
indicated in A.The sections indicate where the neural tube is just beginning to form.
Rhombencephalon
(Hindbrain)
Mesencephalon (Midbrain)
Prosencephalon
(Forebrain)
Optic vesicle
(developing eye)
Developing heart
Prosencephalon
(Forebrain)
Mesencephalon
(Midbrain)
Rhombencephalon
(Hindbrain)
Frontal view
Spinal cord
FIGURE 34-6.
Frontal and
lateral views of a 5-week-old
embryo showing the brain
vesicles and three embryonic
divisions of the brain and brain
stem.
