. Animal biology. Biology; Zoology; Physiology. COORDINATION 201 come tlie seat of those specializations which have evolved into the nervous system and sense organs. Such is the case in all forms from the lowest to the highest, and thus the development and comparative anatomy of the nervous system of Vertebrates, in particular, affords strong evidence of the genetic continuity of the whole series. In the development of a Vertebrate, the first indication of the nervous system is a longitudinal groove in the ectoderm along the dorsal surface, which soon becomes converted into a tube by the Ventr
. Animal biology. Biology; Zoology; Physiology. COORDINATION 201 come tlie seat of those specializations which have evolved into the nervous system and sense organs. Such is the case in all forms from the lowest to the highest, and thus the development and comparative anatomy of the nervous system of Vertebrates, in particular, affords strong evidence of the genetic continuity of the whole series. In the development of a Vertebrate, the first indication of the nervous system is a longitudinal groove in the ectoderm along the dorsal surface, which soon becomes converted into a tube by the Ventral nerve cord at ganglion Motor fiber ending in longitudinal muscle Motor neuron cell body Sensory fibers Body wall. Body cavity Longitudinal muscle Circular muscle Epithelium Sensory cells Fig. 139. — Diagram of primary sensory and motor neurons of the ventral nerve cord of an Earthworm, showing their connections with the skin and the muscles to form a simple reflex arc. See Fig. 61. apposition and, finally, the fusion of its edges. This neural tube then becomes separated from, and sinks below the surface ecto- derm, and in time forms the central nervous system consisting of the brain and spinal cord. As development proceeds, out- growths from the central nervous system establish the peripheral and the autonomic nervous systems, so that structurally as well as physiologically the whole nervous system represents a unit; a single organ, as it were, which secondarily becomes closely iden- tified here and there with sense organ, muscle, or gland, as the case may be. (Figs. 142, 174.) The first marked structural modifications in the developing central nervous system of Vertebrates are two constrictions of the enlarged anterior end of the neural tube, which establish the three primary brain vesicles: fore-brain, mid-brain, and hind- brain. Thus very early in embryonic development, one end of the neural tube is molded into the brain, leaving the rest to become the spinal cord. (Fig
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