. Cunningham's Text-book of anatomy. Anatomy. 622 THE NERVOUS SYSTEM. the roof. Into this secondarily formed caudal extension of the chorioid lamina the invagination that commenced in the roof of the foramen interventriculare extends until it reaches the inferior extremity of the deep cleft separating the cerebral hemisphere from the thalamus (Fig. 548, A). Below this point the thalamus remains in uninterrupted continuity with the floor of the cerebral hemisphere (Cs), winch is becoming thickened to form the corpus striatum. At a very early stage in the development of the embryo, long before t
. Cunningham's Text-book of anatomy. Anatomy. 622 THE NERVOUS SYSTEM. the roof. Into this secondarily formed caudal extension of the chorioid lamina the invagination that commenced in the roof of the foramen interventriculare extends until it reaches the inferior extremity of the deep cleft separating the cerebral hemisphere from the thalamus (Fig. 548, A). Below this point the thalamus remains in uninterrupted continuity with the floor of the cerebral hemisphere (Cs), winch is becoming thickened to form the corpus striatum. At a very early stage in the development of the embryo, long before there is any sign of the hemisphere vesicles, the ectoderm upon each side of the anterior neuro- pore (see p. 500) becomes thickened to form the area olfactoria (see Fig. 440, D, p. 501). Certain of the epithelial cells in this area become con- verted into bipolar sensory cells, which become specially adapted to be affected by-certain kinds of air-borne chemical stimuli that awaken a consciousness of smell. These cells always remain in situ in the olfactory epithelium, just as the most primitive sensory cells do in Hydra (Fig. 439, p. 497). But other nerve - cells seem to be derived from the area olfactoria which do not remain in the parent epithelium, but become attached to the adjoining part of the neural tube. These cells form the olfactory ganglion, which acts as the receptive organ for the impressions brought into it by the processes of the sensory cells in the olfactory epithelium; and the Fig. Fissura chorioidea. Pallium. -Venlriculus lateralis. Lamina chorioidea venrriculi lateralis. .V^-Foramen inhervenfriculare. Lamina chorioidea venrriculi rerfii. slriarum. Floor plare. 549.—Diagram of a Transverse Section through a Fcetal Brain to show the Invagination of the Roof through each interventricular Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illu
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