. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. Body ill Vertebrates and Artliropods. 247 becomes verticallj prolongeJ in transverse section, and the central canal wiiicli it contains also becomes vertically ; Then follows the differentiation (1) of the epithelium of the central canal, (2) of the grey matter of the cord, and (3) of the internal coating of white matter. " The white matter is apparently the result of a differentiation of the outermost parts of the superficial cells of the cord into longi-
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. Body ill Vertebrates and Artliropods. 247 becomes verticallj prolongeJ in transverse section, and the central canal wiiicli it contains also becomes vertically ; Then follows the differentiation (1) of the epithelium of the central canal, (2) of the grey matter of the cord, and (3) of the internal coating of white matter. " The white matter is apparently the result of a differentiation of the outermost parts of the superficial cells of the cord into longi- tudinal nerve-fibres, which remain for a long period without a medullary sheath The grey matter and the central epithelium are formed by a dift'erentiation of the main mass of the spinal ; There thus appears to be a lack of homology in the histo- logical topography and origin of the nervous system in Chordata as compared with the Annelidan worms and the Arthropods. The relation of the nervous system of Arthropods is con- stant ; after the stomodaium has been formed, commissures from the brain pass down and connect the latter with the subcesophageal ganglion, which is ventral. This relation of the postoesophageal nervous system to the ventral side of the Fi-. Relatious of the nervous system of an embryo Ortliopterous insect to the body-walls : hr, braili; slxj^ siiboesopliageal ganglion; mj, nervous cord; st, stomodreum; ;)>•, proctodseum; mc, nialpighian tubes; mesen, mid-intestine : ht, heart: md, mandibles; mx, inx', 1st and 2nd maxillae. From Ayers, with changes. body is as constant as the disposition of the ventral surface of the embryo of Insects before the revolution of the embryo, or of the embryos of Annelid worms and Crustacea. The position of the Arthropod embryo is the reverse of that of Vertebrates. The vertebrate disposition of the priiuitive nervous system is also seen in the embryo Tunicate (figs. 1, 2). Mori^hology.—The brain of the Arthropo
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