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.

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 knowledge+.

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 in

ternal 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 germcolls 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 cach 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 putriment, 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 scat 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

on the processes of development in the Hydra, appear to be the chief conditions influencing that modification of the generative process by which a small portion only of the Hydra is taken into the system of the new individual, instead of one-half of the body, as in the case of the Monad. So insignificant is the distinction between gemmation and spontaneous fission; the essential condition of both being, as in the development of the ovum, the presence of the pellucid nucleus of a derivative germ-cell, as the centre from which all the processes in the formation of the new individual radiate.

The Hydra propagates by ova as well as by buds. Certain of the retained fertile germ-cells multiply themselves, and coalesce to form a larger central cell, surrounded by others of the ordinary smaller size, the exterior of which are metamorphosed into a chorion. Certain other germcells are converted into sperm-cells, and develope spermatozoa. The ova are extruded and fertilized by these: each constitutes the first term of a new series of life and developes a Hydra, which retains however a large proportion of unchanged cells in its composition. Accordingly this Hydra may propagate by buds, and the Hydra so developed may propagate again by ova, and these two kinds of generation may alternate indefinitely.

In most of the marine hydriform polypes, the delicate tissue of the body is supported in the waves and breakers, and protected from the briny element by an external horny integument. Were such a polype to propagate by gemmation, and the external crust to grow with the growth of the bud, and expand to protect the soft digestive sac of the new polype, an apparently compound animal would result from the number of individuals so held together. Such is the nature of a vast family of marine zoophytes which our Lyncean Ellis has so accurately described and beautifully figured as 'Corallines*,' and which are grouped

* An Essay towards a Natural History of Corallines, &c., 4to. 1755.