bouring cells are often established; and (4) of cell contents, which can be chemically analysed, and which are products of the vital activity rather than parts of the living substance, such as pigment, fat, and glycogen or animal starch.

The growth of all multicellular animals depends upon a multiplication of the component cells. Like organisms, cells have definite limits of growth which they rarely exceed; giants among the units are rare. When the limit of growth is reached the cell divides.

The necessity for this division has been partly explained by Spencer and Leuckart. If you take a round lump of dough, weighing an ounce, another of two ounces, a third of four ounces, you obviously have three masses successively doubled, but in doubling the mass you have not doubled the surface. The mass increases as the cube, the surface only as the square of the radius. Suppose these lumps alive, the second has twice as much living matter as the first, but not twice the surface. Yet it is through the surface that the living matter is fed, aerated, and purified. The unit will therefore get into physiological difficulties as it grows bigger, because its increase of surface does not keep pace with its increase of mass. Its waste tends to exceed its repair, its expenditure gains on its income. What are the alternatives? It may go on growing and die (but this is not likely), it may cease growing at the fit limit, it may greatly increase its surface by outflowing processes (which thus may be regarded as life-saving), or it may divide. The last is the usual course. When the

unit has grown as large as it can conveniently grow, it divides; in other words, it reproduces at the limit of growth, when processes of waste are gaining on those of construction. By dividing, the mass is lessened, the surface increased, the life continued.

But although we thus get a general rationale of celldivision, we are not much nearer a conception of the internal forces which operate when a cell divides; for in most cases the process is orderly and complex, and is somehow governed by the behaviour of the nucleus. Few results of the modern study of minute structure are more

marvellous than those which relate to dividing cells. From Protozoa to man, and also in plants, the process is strikingly uniform. The nucleus of the cell becomes more active, the coil or network of threads which it contains is undone and takes the new and more regular form of a spindle or barrel. The division is most thorough, each of the two daughter-cells getting an accurate half of the original nucleus. Recent investigators, moreover, assert

that from certain centres in the cell-substance an influence is exerted on the nuclear threads, and they talk of an archoplasm within the protoplasm, and of marked individuality of behaviour in the nuclear threads.

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From the cell as a unit element we penetrate to the protoplasm which makes it what it is. Within this we discern an intricate network, within this, special centres of force "attractive spheres" and "central corpuscles," or an archoplasm" within the protoplasm! We study the nucleus, first as a simple unit which divides, years afterwards as composed of a network or coil of nuclear threads which seem ever to become more and more marvellous, "behaving like little organisms." We split these up into "microsomata," and so on, and so on. But we do not catch the life of the cell, we cannot locate it, we cannot give an account of the mechanics of cell-division. It is a mystery of life. After all our analysis we have to confess that the cell, or the protoplasm, or the archoplasm, or the chromatin threads of the nucleus, or the "microsomata" which compose them, baffle our analysis; they behave as they do because they are alive. Were we omniscient chemists, such as Laplace imagined in one of his speculations, and knew the secret of protoplasm, how its touch upon the simpler states of matter is powerful to give them life, we should but have completed a small part of those labours that even now lie waiting us; what further investigations will present themselves we cannot tell.

CHAPTER XII

THE LIFE-HISTORY OF ANIMALS

1. Modes of Reproduction-2. Divergent Modes of Reproduction— 3. Historical-4. The Egg-Cell or Ovum -5. The MaleCell or Spermatozoon-6. Maturation of the Ovum-7. Fertilisation - 8. Segmentation and the first stages in Development-9. Some Generalisations—The Ovum Theory, the Gastraa Theory, Fact of Recapitulation, Organic Continuity

IN his exercitation on the efficient cause of the chicken," Harvey (1651) confesses that "although it be a known thing subscribed by all, that the fœtus assumes its original and birth from the male and female, and consequently that the egge is produced by the cock and henne, and the chicken out of the egge, yet neither the schools of physicians nor Aristotle's discerning brain have disclosed the manner how the cock and his seed doth mint and coine the chicken out of the egge." The marvellous facts of growth are familiar to us -the sprouting corn and the opening flowers, the growth of the chick within the egg and of the child within the womb; yet so difficult is the task of inquiring wisely into this marvellous renewal of life that we must reiterate the old confession: "ingratissimum opus scribere ab iis quæ, multis a natura circumjectis tenebris velata, sensuum lucis inaccessa, hominum agitantur opinionibus."

1. Modes of Reproduction.-The simplest animals divide into two or into many parts, each of which becomes a full-grown Protozoon. There is no difficulty in understanding

why each part should be able to regrow the whole, for each is a fair sample of the original whole. Indeed, when a large Protozoon is cut into two or three pieces with a knife, each fragment is often able to retain the movements and life of the intact organism. Among the Protozoa we find some in which the multiplication looks like the rupture of a cell which has become too large; in others numerous buds are set free from the surface; in others one definitely-formed bud (like an overflow of the living matter) is set free; in others the cell divides into two equal parts, after the manner of most cells; and numerous divisions may also occur in rapid succession and within a cyst, that is, in limited time and space, with the result that many "spores" are formed. These modes of multiplication form a natural

series.

In the many-celled animals multiplication may still proceed by the separation of parts; indeed the essence of reproduction always is the separation of part of an organism to form or to help to form a new life. Sponges bud profusely, and pieces are sometimes set adrift; the Hydra forms daughter polypes by budding, and these are set free; sea-anemones and several worms, and perhaps even some star-fishes, multiply by the separation of comparatively large pieces. But this mode of multiplication--which is called asexual-has evident limitations. It is an expensive way of multiplying. It is possible only among comparatively simple animals in which there is no very high degree of differentiation and integration. For though cut-off pieces of a sponge, Hydra, sea-anemone, or simple worm may grow into adult animals, this is obviously not the case with a lobster, a snail, or a fish. Thus with the exception of the degenerate Tunicates there is no budding among Vertebrates, nor among Molluscs, nor among Arthropods.

The asexual process of liberating more or less large parts, being expensive, and possible only in simpler animals, is always either replaced or accompanied by another method that of sexual reproduction. The phrase “sexual reproduction" covers several distinct facts: (a) the separa

tion of special reproductive cells; (b) the production of two different kinds of reproductive cells (spermatozoa and ova), which are dependent on one another, for in most cases an ovum comes to nothing unless it be united with a male-cell or spermatozoon, and in all cases the spermatozoon comes to nothing unless it be united with an ovum; (c) the production of spermatozoa and ova by different (male and female) organs or individuals.

(a) It is easy to think of simple many-celled animals being multiplied by liberated reproductive cells, which differed but little from those of the body. But as more and more division of labour was established in the bodies of animals, the distinctness of the reproductive cells from the other units of the body became greater. Finally, the prevalent state was reached, in which the only cells able to begin a new life when liberated are the reproductive cells. They owe this power to the fact that they have not shared in making the body, but have preserved intact the characters of the fertilised ovum from which the parent itself arose.

(b) But, in the second place, it is easy to conceive of a simple multicellular animal whose liberated reproductive cells were each and all alike able to grow into new organisms. In such a case, we might speak of sexual reproduction in one sense, for the process would be different from the asexual method of liberating more or less large parts. But yet there would be no fertilisation and no sex, for fertilisation means the union of mutually dependent reproductive cells, and sex means the existence of two physiologically different kinds of individuals, or at least of organs producing different kinds of reproductive cells. We can infer from the Protozoa how fertilisation or the union of the two kinds of reproductive cells may have had a gradual origin. For in some of the simplest Protozoa, e.g. Protomyxa, a large number of similar cells sometimes flow together; in a few cases three or more combine; in most a couple of apparently similar units unite; while in a few instances, e.g. Vorticella, a small cell fuses with a large one, just as a spermatozoon unites with an ovum.