57680.67 ON THE ARCHETYPE AND HOMOLOGIES OF THE VER TEBRATE SKELETON. 8vo. 1848. Two Folding Plates and Twenty-eight Woodcuts. Price 10s. ON THE NATURE OF LIMBS. 8vo. 1849. Three Plates and Eleven Woodcuts. Price 6s. HARVARD ON PARTHENOGENESIS. THERE is a natural and irrepressible tendency in the human mind to penetrate the mystery of the beginning of things, and above all that of the origin of living things, involving our own origin. But it is plainly denied to finite understandings to ascend to the very beginning, and to comprehend the nature of the operation of the First Cause of anything. And perhaps the best argument from reason for a future state and the continued existence of our thinking part, is afforded by the fact of our being able to conceive the pos. sibility of the enjoyment of such knowledge, and the consequent yearning to possess it,-that μávrevμa Ti of Plato, or parturient vaticination of some higher knowledge which cannot be fulfilled in the present state of our existence. The ablest endeavours here to penetrate to the beginning of things do but carry us, when most successful, a few steps nearer that beginning, and then leave us on the verge of a boundless ocean of the unknown truth, dividing the secondary or subordinate phænomena in the chain of causation from the great First Cause. The brief record of creation in the Sacred volume leaves us to infer that certain plastic and spermatic qualities of common matter were operative in the production of the first organized Beings of this planet. "The earth brought forth grass and herb yielding seed after his kind, and the tree yielding fruit whose seed is in itself." "The waters brought forth abundantly the moving creature that hath life;" and "the earth brought forth the living creature after his kind, cattle, and creeping thing, and beast of the earth." But of our own species it is written, "God created man after his own image, in the image of God created he him; male and female created he them." And "God said unto them, Be fruitful, and multiply and replenish the earth." (Gen. i. 27 and 28.) Since that first fiat went forth, the propagation of the species of plants, animals and mankind has been left to the operation of certain natural secondary causes, which we sum up as the 'act of generation.' Botanists and physiologists have observed and progressively analysed the phænomena until they have reduced them to a great degree of simplicity, the essential conditions being the same, or closely similar, in both realms of organic nature. With regard to the animal kingdom, the generation of which here concerns us, the essential conditions of the act appear to be a nucleated cell, and the product of a nucleated cell, with the combination of the two: the nucleated cell is the 'germinal vesicle,' and is the essential part of the ovum; the other nucleated cell is the 'sperm-cell,' and its product is the spermatozoon. It is essential to the development of the germ that the ovum receive the matter of the spermatozoon: it is then said to be impregnated. The phænomena that thence ensue are essentially the same up to a certain point in all animals, and consist in the formation of a germ-cell (Pl. I. fig. 4, c), and its propagation of a numerous offspring (fig. 10) at the cost of the germ-yelk (fig. 4, a), by a series of reiterated spontaneous divisions. The right and clear comprehension of the purpose of this process, or the object effected by it, is essential to the elucidation of the nature and relations of the subsequent modifications and varieties in the course of development. The progeny of the 'primary impregnated germ-cell' may be called secondary' or 'derivative impregnated germ-cells,' and the whole is the 'germmass.' The progeny of the impregnated germ-cell resemble their parent, with a diminution of size, to a certain stage of descent, when they may be ultimately reduced to their essential parts or nuclei (fig. 11). When they cease to exist as germ-cells or nuclei of such, either by coalescing with others or by liquefaction, they do not lose their vitality: as individuals, indeed, they may be said to die, but by their death they minister to the life of a being higher than themselves (e. g. figs. 12, 13). They combine to construct its tissues or dissolve and impart properties to its fluids; these metamorphoscs being mysteriously governed by a plastic nature or mode of force operating unconsciously upon the matter, but according to a law of order and harmony, and ·to a fore-ordained and definite end, resulting in a distinct and specific form of animal adapted by its organization for a particular sphere of existence, and forming a more or less valuable, but not, as once was thought, an essential link in the great chain of organic life. Not all the progeny of the primary impregnated germcell are required for the formation of the body in all animals: certain of the derivative germ-cells may remain unchanged and become included in that body which has been composed of their metamorphosed and diversely combined or confluent brethren: so included, any derivative germ-cell or the nucleus of such may commence and repeat the same processes of growth by imbibition, and of propagation by spontaneous fission, as those to which itself owed its origin; followed by metamorphoses and combinations of the germ : |