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Exterior of brain of human embryo of five weeks

From model by Wilhelm His (1831-1904).

The Diencephalon.—From the alar lamina of the diencephalon, the thalamus, metathalamus, and epithalamus are developed. The thalamus (Figs. 650 to 654) arises as a thickening which involves the anterior two-thirds of the alar lamina. The two thalami are visible, for a time, on the surface of the brain, but are subsequently hidden by the cerebral hemispheres which grow backward over them. The thalami extend medialward and gradually narrow the cavity between them into a slit-like aperture which forms the greater part of the third ventricle; their medial surfaces ultimately adhere, in part, to each other, and the intermediate mass of the ventricle is developed across the area of contact. The metathalamus comprises the geniculate bodies which originate as slight outward bulgings of the alar lamina. In the adult the lateral geniculate body appears as an eminence on the lateral part of the posterior end of the thalamus, while the medial is situated on the lateral aspect of the mid-brain. The epithalamus includes the pineal body, the posterior commissure, and the trigonum habenulæ. The pineal body arises as an upward the evagination of roof-plate immediately in front of the midbrian; this evagination becomes solid with the exception of its proximal part, which persists as the recessus pinealis. In lizards the pineal evagination is elongated into a stalk, and its peripheral extremity is expanded into a vesicle, in which a rudimentary lens and retina are formed; the stalk becomes solid and nerve fibers make their appearance in it, so that in these animals the pineal body forms a rudimentary eye. The posterior commissure is formed by the ingrowth of fibers into the depression behind and below the pineal evagination, and the trigonum habenulæ is developed in front of the pineal recess.

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Gray H. Anatomy of the human body. (1918) Philadelphia: Lea & Febiger.

Cite this page: Hill, M.A. (2024, June 25) Embryology Gray0652.jpg. Retrieved from

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current08:22, 20 May 2012Thumbnail for version as of 08:22, 20 May 2012698 × 700 (103 KB)Z8600021 (talk | contribs)==Exterior of brain of human embryo of five weeks== From model by His. :Week 5 Brain: Exterior view | Interior view {{Gray Anatomy}} Category:Human Category:Historic Embryology [[Category:Week 5]