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Somatic efferent (XII)

Special visceral efferent (IX, X, XI)

Figure 19.18 A. Dorsal view of the floor of the fourth ventricle in a 6-week embryo after removal of the roof plate. Note the alar and basal plates in the myelencephalon. The rhombic lip is visible in the metencephalon. B and C. Position and differentiation of the basal and alar plates of the myelencephalon at different stages of development. Note formation of the nuclear groups in the basal and alar plates. Arrows, path followed by cells of the alar plate to the olivary nuclear complex. The choroid plexus produces cerebrospinal fluid.

External granular layer

Rhombic lip

4th ventricle

4th ventricle

Roof plate

Rhombic lip

Basal Plates Metencephalon

Pontine nuclei

Somatic efferent (VI)

Figure 19.19 Transverse section through the caudal part of the metencephalon. Note the differentiation of the various motor and sensory nuclear areas in the basal and alar plates, respectively, and the position of the rhombic lips, which project partly into the lumen of the fourth ventricle and partly above the attachment of the roof plate. Arrows, direction of migration of the pontine nuclei.

Pontine nuclei

Somatic efferent (VI)

Roof plate

Somatic afferent

Special visceral afferent

General visceral afferent

General visceral efferent Special visceral efferent (V and VII)

Figure 19.19 Transverse section through the caudal part of the metencephalon. Note the differentiation of the various motor and sensory nuclear areas in the basal and alar plates, respectively, and the position of the rhombic lips, which project partly into the lumen of the fourth ventricle and partly above the attachment of the roof plate. Arrows, direction of migration of the pontine nuclei.

19.18 B). The two combined are known as the tela choroidea. Because of active proliferation of the vascular mesenchyme, a number of saclike invaginations project into the underlying ventricular cavity (Figs. 19.18C and 19.20D). These tuftlike invaginations form the choroid plexus, which produces cerebrospinal fluid.

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