VERTEBRATA, the highest and most important of the animal sub-kingdoms, charac terized by the universal presence of a backbone, composed of a varying number of small bones called vertebra: (see SKELETON and SPINAL COLUMN), which at once serve for the general support of the other parts, and for the protection of the central part of the nervous system (the brain and spinal cord) in a closed cavity in the interior.
We shall notice lirst the developmental and then the structural peculiarities of the vertebrates. Like the members of the other sub-kingdoms, the vertebrates begin in a semi-fiuid nitrogenous substance called plasma, which separates itself (or differentiates, as it is scientifically termed) into albumen, fibrin, primary membrane (the lemma of Owen), nuclei, and cells, in which form, says the above-named physiologist, " the indi viduality of the new organism first dawns as a nucleated germ-cell, or germinal vesicle." The formation of yolk by the evolution of albuminous granules and oil-particles from the plasma, and the development of an outer layer of membrane, complete the unim pregnated egg. For further development, another principle, the spermatozoon, or product of the sperm-cell, is required. Its reception by the egg is followed by the forma tion of a germ-mass, which is formed by incorporates divisions, cleavages, or segmenta tions of the impregnated center, which ncorporates more or less of the yolk. Thus far there is no difference between the vertebrate and invertebrate germ. The next step, to use the words of professor Owen, " impresses upon the nascent being its vertebrate type." As has been shown in the article DEVELOPMENT OF THE EMBRYO, the parietal portion of the germ becomes raised up on each side into a ridge, so that a long groove or furrow is formed between these parallel ridges (see figs. 6, 7, 8 in DEVELOPMENT); and the margins of these subsequently uniting with one another, constitute a tube, in the interior of which the vertebrate cerebrospinal nervous centers are developed. In the mean time, the margins of the germs extend downward over the yolk till they meet and form the abdominal cavity. Hence in the vertebrates there are developed from the chorda dor sals, or notochord (see DEVELOPMENT), pair of plates neurad,'* to inclose the ner vous axis, and a pair of plates f to inclose the vascular axis and organs of vegetative life. Flesh and skin co-extend with the inclosing plates. This formation of
two distinct parallel cavities—neural and htemal—under symmetrical guidance, in the vertical or neuro-hteinal' direction, with a repetition of parts on the right and left sides, establishing transverse or bi lateral' symmetry, constitutes the chief developmental characteristics of the vertebrate animal."—Owen's Anatomy of Vertebrates, vol. i. p. 2. The accompanying diagrams, which we have borrowed front professor Huxley's Ele ments of Comparative Anatomy, may tend to render this subject more intelligible. In the invertebrates, merely a single saccular or tubular investment is formed, which incloses all the viscera; so that, provided we select one high enough to possess a heart and nervous system—the transverse and longitudinal sections would be represented by 1 and 2, while P represents the parietes, of wall of the body, A the alimentary canal, M the mouth, H the heart, and N the nervous centers. "It will be observed," says professor Huxley, " that the alimen• tiny canal is in the middle, the principal centers of the nervous system upon one side of it, and the heart upon the other.
In none of these animals, again, would you discover in the embryonic state any partition formed by the original external parietes of the body between the nervous centers and the alimentary canal."—Op.
cit., p. 59. But the vertebrate, after it has passed through its very earliest stages of development, is, as we have seen, not a single, but a double tube; and the "two tubes are separated by a partition, which was, primatively, a part of the external parietes of the body, but which now lies in a central position between the cere bro-spinal nervous centers and the ali mentary canal. Hence a transverse see tiou of any vertebrated animal may be represented diagramatically •y fig. 3, where, for the most part, the letters have She same signification as in the foregoing case, but where P denotes the 'second or cere.