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 metamorphoses 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

masses so produced, which concur to the development of another individual; and this may be, or may not be, like that individual in which the secondary germ-cell or germmass was included.

It has been found that in proportion as the subjects of anatomical investigation descend in the scale of animal life, the number of the derivative germ-cells and nuclei which retain their individuality and spermatic power is greater, and the number of those that are metamorphosed into tissues and organs less.

Cells predominate in the tissues of the vegetable kingdom, the lower members of which consist exclusively of them, and have been thence called 'plantæ cellulares :' the lowest of all consist of a single nucleated cell.

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The animal kingdom starts from the same elementary beginning: a cell-wall forms the smooth elastic and contractile integument of the Gregarina*: a fluid with granules, and a firm nucleus which sometimes contains one or more nucleoli, the ordinary cell-contents,-are the sole representatives of organs or viscera. Yet the power of the Gregarine to live and grow independently by assimilating foreign nutriment, the vital contractility of their tegumentary tunic, their chemical composition and their definite forms, with such well-marked specific characters, in a few instances, as the Greg. brevirostris and Greg. Sieboldii present, render their interpretation by Kölliker as a low and primitive form of parasitic animal, the most accordant with actual physiological and zoological knowledget.

The Gregarina is a single-celled animal, which differs from the single-celled plant by the vital contractility of its tissue, and the solubility of its cell-wall in acetic acid. Devoid of mouth, stomach, or any other organ properly so

* A genus of microscopic parasites which infest gregariously the internal cavities and canals of insects and worms.

↑ Kölliker, 'Ueber die Gattung Gregarina,' Zeitschrift für Wissenschaftliche Zoologie, Bd. i. p. 10, 1848.

called, it reduces our definition of an animal to the difference indicated in the preceding comparison.

The Polygastric Infusoria exhibit the next step in the progress of individualizing a higher independent embodiment of animal life. A firm central nucleus in which, as in the Gregarina, resides the self-repellent property of spontaneous division, indicates, however, their essential character as animated cells. But the granular contents have been developed into secondary cells; and some of these have combined and coalesced to form special organs, such as cavities for digestion, pulsatile cells for circulating a clear plasmatic fluid, an irritable and contractile integument beset with vibratile cilia: yet a large proportion of the contents of this modified primary cell-wall consists of unchanged secondary cells.

In the freshwater polype, Hydra, an external layer of cells is partially condensed into an integument, and an internal layer modified to form the gastric secerning villi: in the tentacula a greater proportion of the derivative cells have been metamorphosed into the muscular bands, the nodosities, the prehensile darts and tactile cilia. But the chief point that I have now in view is to draw attention to the large proportion of retained and unaltered nucleated cells and nuclei, which are identical in all recognizable characters with the progeny of the primary impregnated germ-cell, and which are ready, therefore, when favourable circumstances concur, to repeat the acts of assimilation and spontaneous fission, and, each individually, thus to lay the basis of a new polype.

A large proportion of the derivative germ-cells is retained unchanged in the compound hydriform polypes and in the parenchymatous Entozoa: a smaller proportion in the Acalephæ and cavitary Entozoa. Derivative germcells are aggregated in the last segment of the Nais, and of the young of other Annelides. We find derivative germ-cells, and masses of nuclei like those resulting from

the final subdivision of germ-cells, retained unchanged at the filamentary extremities of the branched uterus forming the ovaria of the larval Aphides.

In most of the lower classes of animals the course of development is temporarily arrested at certain stages, though growth may go on; the embryo moving and feeding, and perhaps propagating, as if it were a completed individual, usually under a form very different from that which itself or its progeny are destined ultimately to assume; whence these arrested forms have been termed 'Larvæ,' the true lineaments of the fully developed form being hidden, as it were, beneath a mask.

The earlier the individual in any of these larval stages may have been arrested from the commencement of its development from the germ-mass, the greater is the proportion of the derivative impregnated germ-cells and nuclei that continue unchanged in its constitution: and the result of the retention of these, in the hydriform larvæ of Acalephes, e. g., is the exercise, as in the mature freshwater Hydra, by one or more of such retained progeny of the primary impregnated germ-cell, of the powers derived from the legacy of the portion of the spermatic virtue which they received from their parent nucleated cell.

In the Polygastria, e. g. when favourable influences of warmth, light and abundant nutriment concur, a central body, which represents the nucleus of the impregnated germ-cell, sets on foot the special act of assimilation and spontaneous fission; and its divisions seem to repel each other to positions equidistant from each other, and from the pole or end of the body to which they are nearest.

The influence of these distinct centres of assimilation is to divert the flow of the plasmatic fluid from a common course through the body of the Polygastrian to two special courses about those centres. So much of the primary developmental processes is renewed, as leads to the insulation. of the sphere of the influence of each assimilative centre

from that of the other by the progressive formation of a double party-wall of integument, attended by progressive separation of one party-wall from the other, and by concomitant constriction of the body of the Polygastrian, until the vibratile action of the superficial cilia of each separating moiety severs the narrowed neck of union, and they become two distinct individuals. An eye-speck, a pulsatile sac, a proboscis, or whatever organ may be required to complete the specific characters of the particular Polygastrian, are likewise developed, and the individualization of each moiety is thus completed.

This mode of propagation is termed 'spontaneous fission.' In the freshwater polype the progeny of the primary impregnated germ-cell retained unaltered in the body, may set up, under favourable stimuli of light, heat and nutriment, the same actions as those to which they owed their own origin: certain of the nucleated cells do set up such actions, those, e. g., in the Hydra fusca, which are aggregated near the adhering pedicle or foot; and the result of their increase by assimilation and multiplication is to push out the contiguous integument in the form of a bud, which becomes the seat of the subsequent processes of growth and development: a clear cavity or centre of assimilation is first formed, which soon opens into the stomach of the parent: but the communication is afterwards closed, and the young Hydra is ultimately cast off from the surface of the parent.

This mode of propagation is termed 'Gemmation.' It differs from the development of the Hydra ab ovo inasmuch as the impregnated germ-cell, which set on foot the process, is derivative and included in the body of the adult, instead of being primary and included in a free ovum. But the germ-cell is the essential part of the ovum, and the chorion an accessory and non-essential part.

The very small size in relation to the entire body, and the superficial position, of the derivative germ-cell which takes