. Chordate anatomy. Chordata; Anatomy, Comparative. A. NEUROSENSORY CELLS OF AN^EUD. B NEUROSENSORY CELLS OF OLFACTORY MEM- BRANE OF MAN. C. PRIMARY SENSORY CELL D. SECONDARY SENSORY WITH FREE NERVE CELLS CONVEY IMPULSES TERMINATION. TO THE PRIMARY SENSORY CELL. Fig. 348.—Stages in the hypothetical evolution of secondary sense cells. A and B, diagrams of neurosensory cells in invertebrate (A) and vertebrate (B). C, a sensory cell (neuron) with free nerve termination. D, secondary sense cells convey impulses to the primary sensory cell. The series assumes that the definitive receptor cells are
. Chordate anatomy. Chordata; Anatomy, Comparative. A. NEUROSENSORY CELLS OF AN^EUD. B NEUROSENSORY CELLS OF OLFACTORY MEM- BRANE OF MAN. C. PRIMARY SENSORY CELL D. SECONDARY SENSORY WITH FREE NERVE CELLS CONVEY IMPULSES TERMINATION. TO THE PRIMARY SENSORY CELL. Fig. 348.—Stages in the hypothetical evolution of secondary sense cells. A and B, diagrams of neurosensory cells in invertebrate (A) and vertebrate (B). C, a sensory cell (neuron) with free nerve termination. D, secondary sense cells convey impulses to the primary sensory cell. The series assumes that the definitive receptor cells are secondary. The possibility that the neurosensory cells become the definitive sense cells and that the sensory nerve is secondary must be admitted. (Redrawn after Fritz Kahn, "Der Mensch," Albert Miiller, Zurich.) end between tactile cells, or become surrounded by a connective-tissue capsule. A definitive evolutionary stage is attained when a secondary sensory cell becomes the receptor element by means of which a stimulus is transferred to the dendrites of a neuron. The receptor cells of the taste-buds and the hair-cells of the cochlea represent a final stage of this sort. The substitution of a secondary sensory cell for the primary neurosensory one presents a problem which has never been satisfactorily solved. That such an evolutionary change has occurred seems indisput- able, but transitional stages are wholly conjectural. Among the factors which have led to the formation of the sense organs of higher animals may be noted the tendency of the neurosensory. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son & Co.
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