force have been set up in place of one, and the plastic power has arranged the organic matter around each of these centres, as it was around the single nucleus of which they are

of the germ-yelk has not been seen in the ova of other animals, it is because hitherto only the coarser phænomena of such division of the germinal body in Medusa, Mollusca, Fishes, Frogs, &c. have been noticed. In Dr. Barry's observations however on the development of the germmass in the pellucid ova of the rabbit, phænomena closely analogous to those described by Siebold and Bagge in the ovum of the entozoon were detected.

In reflecting on the phænomena in the monad and the ovum-that a central something is first established, and the consequence of that -I have been led to draw the same conclusion with respect to both, and to regard the establishment of the special centre as the cause of the confluence of the parts around it; and I call it 'a centre of attractive and assimilative force.' Since the pellucid centre of the germinal body has not divided from the necessity of endowing the moiety to be separated by the subsequent fission with a particular organ required for its individual completeness, I infer that the same preliminary act in the monad was not solely for the purpose of providing its separated moieties with their respective testes, but that it had a higher significance.

As the pellucid centre in the ovum is the result of impregnation or of the reception of the matter of the spermatozoon, so it may be concluded that the nucleus of the monad is of a nature similar to, if not identical with, that of the spermatozoon. It was doubtless a gross view of its nature and analogies to regard it as the homologue of the whole preparatory organ of the spermatic fluid, such as is required in the higher animals; because as the germ-cells exist in the body of the Polygastria without the organ called ovarium, so we ought to expect that the essential matter of the sperm would likewise exist without a special testicular envelope.

The objection to Ehrenberg's determination of the nucleus as the 'testis,' that it has never been observed to produce spermatozoa, is akin to that which has been opposed to his determination of the ova, viz. that the young have never been seen to quit them and leave the shell behind. Neither, of these objections will apply to the view of the nucleus as the essential matter of the sperm, and to the germ-cells as the essential elements of ova.

A filamentary spermatozoon is doubtless a very general form of the essential matter of the sperm: but in tracing the modifications of the spermatozoa from mammalia down the scale of animal life we find them gradually reduced to the head or nuclear part, and discern in the vibratile caudal appendage an accessory relating to the passage of the

the divisions. The same phænomena occur in the development of the cells during the growth of the conferval filament and of other cryptogamic plants.

fertilizing principle to the germ-cell, rather than to its essential operations when arrived there.

The best microscopical examinations of the spermatozoa show "that the spermatozoa are everywhere void of a special organization, and consist of an uniform homogeneous substance which exhibits a yellow amber-like glitter.”—Art. SEMEN, Cyclop. of Anatomy, vol. iv. p. 502. The nucleus of the Polygastrian offers the closest resemblance to this character of tissue. And perhaps it may not be out of place here to notice the close analogy of the modification of form which the nucleus of some of the larger Polygastria, Stentor Roeselii e. g., presents to the spirally disposed elongated head of the spermatozoon in the Torpedo, Pelobates, and the Passerine birds.

The reception of the matter of the spermatozoon by the germ-cell is the essential preliminary to the primary processes of its spontaneous fissions: and when we see these fissions governed in the germ-cell of the Ascaris by the act of impregnation, followed by the appearance of a pellucid nucleus in the centre of the opake and altered germ-cell, and when we further see its successive fissions governed by the preliminary division of the pellucid centre, are we not naturally led to infer that that centre is the seat or the chief seat of the spermatic principle which the germ-cell has received? The mind must either be a mere passive recipient of these phænomena, or we must reason upon them and ask ourselves their meaning. The most probable signification of the appearance of the pellucid centre and of the initiative which it takes in the subsequent changes may be that which I have just explained: but this is certain, that the analogy between these phænomena in the multiplication of the parts of the germ-mass and those of the nucleus in the multiplication of the monads is so close, that one cannot reasonably suppose that the nature and properties of the nucleus of the impregnated germ-cell and that of the monad can be different.

Therefore I infer that the nucleus of the Polygastric Animalcules is the seat of the spermatic force: it can only be called 'testis' figuratively it is the essence of the testis. It is the force which governs the act of propagation by spontaneous fission : and if Ehrenberg be correct in viewing the interstitial cell-corpuscles (körnchen) as germ-cells, these essential parts of ova may receive the essential matter of the sperm from the nucleus which is discharged along with them in the breaking up of the monad, which Ehrenberg regards as equivalent to an act of oviposition; and impregnated germ-cells may thus be prepared to diffuse through space and carry the species of Polygastric Animalcules

Wherefore it may be concluded that the presence of the spermatic force is essential to the process of growth by the multiplication of cells, and that the phænomena of the formation of the germ-mass are the provision for the presence of that force. It is only therefore a question of the degree of spermatic force which may suffice for any of the descendants of the primary impregnated germ-vesicle, in order to recommence and repeat the process to which the cell itself owed its origin. The result of such process would be always the same: the formation or accumulation of a germ-mass; that is to say, a mass of cells prepared as the materials upon which the plastic force might operate in the formation and adjustment of the different tissues and organs of a new individual.

What then might be expected to be the conditions of structure essential to the renewal of the process of forming a germ-mass in an individual organism, independently of the primal act of impregnation? Obviously the retention of some of the progeny of the primary germ-vesicle with their inherited spermatic virtue.

Now we have seen that the power of propagating by gemmation and spontaneous fission is in the ratio of the retention of germ-cells, as such, in the constitution of the individual first developed from the primary germ-mass. Plants are more cellular than animals; monads than rotifers; polypes than echinoderms; the tape worms than the round worms; the radiata than the articulata and mollusca; the invertebrata than the vertebrata.

to a distance from the scene of life of the parent. A very peculiar odour has always been recognized as one of the characteristics of the semen, which leads me to quote the following remark by Ehrenberg :"Vast numbers of the Euglena viridis, dying contracted into a ball, form a delicate green pellicle on the water, which first exhales, as during life, a spermatic and then a mouldy odour: sinks during cold, rises during warmth, is decomposed into minute corpuscles and evolves gas, and the mass finally is resolved into a greyish dust which contains very minute ovules without chorion."

In proportion to the number of generations of germ-cells, with the concomitant dilution of the spermatic force, and in the ratio of the degree and extent of the conversion of these cells into the tissues and organs of the animal is the perfection of the individual, and the diminution of its power of propagating without the reception of fresh spermatic force.

In the vertebrate animal the whole of this force originally diffused amongst the cells or nuclei of the germ-mass is exhausted in the development of the tissues and organs of the individual, in the mysterious renovation of the spermatic power in the male by a special organ, and in the development of ova or cells prepared fit for its reception in the female. It now and then happens, even in the highest of Vertebrata-the human species-that an ovarian germ-cell sets up the process of embryonic development, but without sufficient of the spermatic and plastic power to complete even a larval form: some crude materials of the embryo arc the sole result: teeth, it may be, or hair, with irregular amorphous ossifications, such as are met with occasionally in ovarian cysts.

The completion of an embryonic or larval form by the development of an ovarian germ-cell, or germ-mass, as in the Aphis, without the immediate reception of fresh spermatic force, has never been known to occur in any vertebrate animal.

The condition which renders this seemingly strange and mysterious generation of an embryo without precedent coitus possible, is the retention of a portion of the germmass unchanged. One sees such portion of the germ-mass taken into the semitransparent body of the embryo Aphis, like the remnant of the yelk in the chick. I at first thought that it was about to be inclosed within the alimentary canal, but it is not so. As the embryo grows it assumes the. position of the ovarium, and becomes divided into oval masses and inclosed by the filamentary extremities of the

eight oviducts. Individual development is checked and arrested at the apterous larval condition. It is plain, therefore, that the essential condition of the development of another embryo in this larva is the retention of part of the progeny of the primary impregnated germ-cell.

What is really surprising in the phænomena of the Aphides is the potency of the mysterious virtue of the quintessential excretion, which sustaining so great a degree of subdivision, and of dilution with the material incorporated in the successive generation of cells, is nevertheless equal to the renewal and repetition of embryonic development through so many generations.

The explanation of the condition essential to this process is still more clearly afforded by the development of the Distoma tarda as described by Steenstrup. The entire impregnated ovum consists, as in most of the lower Invertebrata, of the germ-cells, which have assimilated the whole of the germ-yelk; the ovum, therefore, grows with the growth of the germ-mass which is formed by the usual processes of imbibition and spontaneous fission producing the multiplication of cells and nuclei: but certain of the outermost of these nuclei liquefy or coalesce, and form an additional layer to the chorion of a contractile or a ciliated structure. The ovum is thus transformed into a subelongate, subdepressed locomotive animal; which contains, however, nothing in its interior except the multiplied offspring of the primitive germ-cell. Here, therefore, we have the condition which I regard as essential to the development of an animal under circumstances which Steenstrup would define as 'alternate generation by the vital powers and by means, of the bodies of the producing individuals,' and which the reviewer who has criticised him would prefer to call a development by a multiplication of cells by a process of continuous growth.' Neither of these definitions explain the condition to which I refer. That condition is the existence in each of the cell-progeny of the primary