thrown at once upon the world, and is dependent for its supply of food upon its own absorbing and assimilating powers; these enable it to multiply itself by fission, sometimes to a vast extent; and thus an elaborate and complex organism (such as a Tree-fern) may be produced.-In the third form of the generative process, which is peculiar to Phanerogamia (or Flowering Plants), there is the same distinction between "spermcells" and "germ-cells;" but the mode in which the action of the former upon the latter is brought about, is very different. The "sperm-cell," which is known as the pollen-grain, and is developed in the anthers of the flower, does not here evolve self-moving filaments, but, when it falls upon the apex of the style, puts forth long tubes, which insinuate themselves down between its loosely-connected tissue, until they reach the ovary at its base. Here they meet with the ovules, which are in reality "germ-cells" imbedded in a mass of nutriment stored up by the parent; and the pollen-tube, entering the micropyle or foramen of the ovule, penetrates into such close approximation to the germ-cell contained within it, that its contents find a ready passage by endosmose into the latter (Fig. 134, c). Here, again, therefore, we have the same essential phenomenon, the intermixture of the contents of the sperm-cell and of the germ-cell, as the condition for the development of the true germ. But this germ, still making its first appearance as a single cell within the ovule, is supplied with nutriment by its parent; and this not merely whilst the ovule remains in connexion with the organism which evolved it, but for some time subsequently, the store laid up around it in the seed being the material at the expense of which its early development takes place. It is not, in fact, until its true leaves have been evolved and its root-fibres have penetrated the soil, which takes place in the act of germination, that it becomes capable of absorbing and assimilating nutriment for itself. As soon, however, as this takes place, the young plant becomes independent of further assistance; and all its subsequent growth is provided for by its own powers. In process of time, its generative apparatus is evolved; and here, too, we find, that the two sets of sexual organs are usually developed in the same organism, it being only a small proportion of Phanerogamia that are diacious, i. e., that have the male or staminiferous flowers, and the female or pistilline, restricted to different individuals.*

781. The history of embryonic Development in Flowering Plants, presents some interesting points of correspondence with that of the higher Animals.-The germ that is developed within the germ-cell (here designated the "embryonic vesicle" of the ovule) as the product of the admixture of its contents with those of the sperm-cell (or pollengrain), is itself a single cell; and the early history of its development closely resembles that which may be observed in all the inferior Plants. In the first place it subdivides into two, each of these into two others, and so on; its first nisus or tendency being to the production, not of the parts which are to be evolved into the stem, roots, leaves, &c., of the perfect plant, but of a leaf-like expansion, which may be likened to the

For a more particular account of the recent discoveries, on which the above account of the Generation of Plants is based, see the Author's "Principles of General and Comparative Physiology," chap. xviii., sect. 2.

frond of the Cryptogamia, and of which the function is only temporary. It is by this organ, the single or double cotyledon, that the nourishment provided in the ovule is absorbed and prepared for the development of the young Plant; the permanent fabric of which, even at the time of the maturity of the seed, forms but a small proportion of the entire embryonic structure. In the act of germination, however, the permanent portions are developed at the expense of the temporary, the plumula and radicle absorbing the nourishment which has been elaborated by the cotyledons; and having fulfilled its transient purpose, and completed its term of life, the first leaf-like expansion withers and dies. The tissues of the young Plant are at first of the simplest possible character; but as the organs characteristic of its adult condition are one after another put forth (always originating in peculiar groups of cells), so do we find that the spiral vessels, woody fibre, &c., characteristic of the higher organisms, gradually make their appearance. Thus we see that even the highest Plants have to pass through conditions closely conformable to those which are permanently shown in the lower; and that the parts which are first formed are destined for only a temporary purpose, that of preparing nourishment for the evolution of more permanent structures. We shall find, in tracing the history of the development of the higher Animals, that exactly the same general fact may be observed, in even a more striking manner; the number of different stages being greater, and a yet larger proportion of the parts first formed having a merely temporary purpose, and being destined to an early decay, as soon as the more permanent parts of the fabric shall have been evolved.

782. Among many of the lower Animals, a multiplication of individuals takes place by a process that closely resembles the budding of Plants; this also must be regarded, not as a proper act of Generation, but as a modification of the ordinary Nutritive process. The same may be said of the powers of reparation, which every animal body possesses in a greater or less degree, but which are by far the most remarkable among the lower tribes; for when an entire member is renewed (as in the Star-fish), or even the whole body is regenerated from a small fragment (which is the case in many Polypes), it is by a process exactly analogous to that which is concerned in the reparation of the simplest wound in our own bodies, and which, as already explained (§ 636), is but a modification of the process that is constantly renewing, more or less rapidly, every portion of their fabric. Although the buds thus produced and separated are usually developed into the likeness of the parent stock, yet this is sometimes not the case, the stock possessing one form, and the bud another, which may be quite different; as when certain fixed composite Zoophytes bud off free-moving solitary Medusa, these last depositing ova, from which the Zoophytic type is regenerated. When, however, this phenomena, to which the name of "alterations of generations" has been given (erroneously, in the Author's opinion),* is carefully examined, it is found that the bud thus detached is really

*For a discussion of this subject, see the Author's "Principles of General and Comparative Physiology," chap. xviii., sect. 1.

the generative apparatus of the parent stock, furnished (it may be) with nutrient and locomotive organs of its own; and that neither can be regarded as a complete organism without the other. Thus, the Medusa contains the proper generative apparatus of the Zoophyte, which developes no other; and the "aggregate" Salpæ that are buddedforth from a kind of stalk in the interior of the "solitary" form, must be regarded as altogether constituting its true generative apparatus, since it never produces any other. In all instances it will be found, that whatever may be the variations which present themselves in the entire history of any species, the immediate product of the true Generative act is always the same.

783. This act, in Animals as in Plants, requires the concurrent action of two sets of organs, evolving "sperm-cells" and "germ-cells" respectively; and it is curious that these should present the closest approach to those of the higher Cryptogamia, rather than to those of Plants above or below them in the scale. The two sets of organs may be united in the same individual, as they are in most Plants; and the ova may be fertilized from the seminal cells of the same being;—as happens in many Zoophytes and in some of the lowest tribes of Mollusca. Or, the two sets of organs being present in each individual, it may not be capable of self-impregnation; but, in the congress of two individuals, each impregnates, and is impregnated by, the other;—as may be observed in the Snail, and many of the higher Molluscs. Or the sexes may be altogether distinct; one individual possessing only the male or spermatic organs; and the other the female, or germ-nourishing apparatus; this is observed in the higher classes of the Radiated, Molluscous, and articulated sub-kingdoms and it is the case in all Vertebrata. 784. The earliest part of the history of Embryonic Development is nearly the same in all Animals; for it consists in the multiplication of the single cell of which the original germ is composed, until a cluster is formed, all the cells of which appear to be in all respects similar to one another. Each of these cells either takes into itself, or draws around it, a portion of the vitellus or yolk, which is the nutrient substance of the ovum; and thus either the whole of this vitellus, or a portion of it, is subdivided into a number of minute spherules, altogether constituting what is known as the "mulberry mass" (Fig. 135). The former seems to be the case, when the grade of development of the organism which is to be formed at the expense of the yolk is very low; whilst the latter plan is followed, when the yolk is destined to afford a prolonged sustenance to the embryo, which attains a high degree of development whilst supported upon it alone. Thus among the Invertebrata generally, we find that the embryo comes forth from the egg in a very simple condition, a large part of its structure having undergone but little change from the state of the "mulberry mass;" and in these, the whole yolk undergoes subdivision. The same is the case, too, in the Batrachian Reptiles, which issue from the egg in a form very different from that into which they are to be subsequently developed; and it is the case even with Mammalia, but for a very different reason, their embryonic structure, formed at the expense of the yolk, being destined to acquire additional material for its full development from a source altogether

different. In the highest Mollusca, however, as also in Fishes, Reptiles, and Birds, the portion of the yolk which undergoes subdivision is comparatively small; and the great mass of the vitellus is destined to be

Successive stages of segmentation in the vitellus of the Ovum of Ascaris acuminata :—A, ovum recently impregnated, the yolk-bag slightly separated from the enveloping membrane; B, first fission into two halves; c, second fission, forming four segments; D, yolk, now divided into numerous segments; E, formation of "mulberry mass" by further segmentation; F, the mass of cells now beginning to show the form of the future worm; G, further progress of its evolution: H, the worm, formed by the conversion of the yolk-cells now nearly mature.

subsequently absorbed into the substance of the germ, by a process analogous to that by which the food of the adult is imbibed. Hence the portion of their yolk which undergoes subdivision, and helps to constitute the "mulberry mass," may be termed the "germ-yolk," whilst the remainder may be designated as the "food-yolk.'

785. When the whole of the yolk is taken into the mulberry mass, the formation of the embryo is usually the result of the progressive metamorphosis of its parts; the cells of the surface being converted into the integument, and those of the inner part into the internal organs. This is the case, for example, in the Intestinal Worm, some of the stages in whose development are shown in Fig. 135. The embryonic condition of many of the organs is frequently retained, at the time when the young animal comes forth from the egg; those parts only being completed which are necessary to enable it to obtain its nutriment.Other organs are subsequently evolved, at the expense of the food thus introduced; and thus a complete change or metamorphosis may take place, in regard alike to external form and to internal structure, between the larval and adult states. Of this phenomenon we have characteristic examples in the groups of Insects and Batrachia; and although it was formerly considered exceptional, it is now known to be the ordinary occurrence among the lower tribes of animals; it being comparatively rare for any of them to come forth from the egg under their adult forms. This change is sometimes obviously gradual, as in the progressive advance of the Tadpole into the condition of the Frog; but it is sometimes apparently sudden, as when the Chrysalis skin is thrown off, and the perfect Insect comes forth. In the latter case, however, the change is really just as gradual as in the former; since the development of the organs characteristic of the perfect Insect is taking place during the whole of the Chrysalis period, to be displayed and brought

into use at its termination. Thus the whole life of the Insect, up to its last change, may be regarded as one of prolonged embryonic development; and the same may be said of that of the Frog, up to the time when its permanent organs are fully evolved.-No such ostensible metamorphosis takes place, however, in any of the animals which are provided with a "food-yolk;" for this supplies that material for the continued development of the embryo within the egg, which is elsewhere to be obtained out of it; and thus the embryo is supported, until it has nearly attained its adult condition, although far from having acquired its adult size. Now in all these cases, it is very interesting to remark that the first nisus is towards an extension of the embryonic mass as a membranous expansion (evidently analogous to the cotyledon of the Flowering Plants, § 781) over the "food-yolk;" in this "germinal membrane," which forms a sort of temporary stomach, blood-vessels are developed, which absorb the prepared nutriment and convey it to the permanent portion of the embryonic structure; and when its function is completed, the store of aliment being exhausted, and the proper nutrient apparatus of the embryo being ready for action, we lose sight of it altogether. We shall find that a similar germinal membrane is formed in the Human ovum, although there is no "food-yolk;" its formation being apparently requisite for ulterior purposes, and the portion of the mul berry mass which gives origin to the permanent part of the embryonic structure being comparatively small.

2. Action of the Male.

786. The share in the Reproductive Function, which belongs to the Male Sex, essentially consists in the formation and liberation of the fertilizing bodies termed Spermatozoa. These are prepared within peculiar cells, as already described (§ 241); and the "sperm cells" are either scattered through the soft parenchyma of the body, as happens among some of the lowest animals; or they are confined to certain parts of it, as in those a little more elevated in the scale; or they are formed within follicles or tubes, clustered together into an organ of a glandular character, known as the Testis. Such an organ is found in all Insects and Mollusca; as well as in Vertebrated Animals. In the first of these classes, it is formed on the general plan of their proper glands ($ 720); being usually composed of tubes, more or less elongated, and sometimes terminating in enlarged follicles. In the Molluscs, on the other hand, it is almost invariably composed of clusters of follicles. In either case, the seminal cells are developed within the tubes or follicles, as are the ordinary secreting cells of the Liver or Kidney within the tubes or follicles of those glands; and their contents are discharged by an excretory duct, which terminates in an organ that conveys them out of the body, either emitting them into the surrounding water (as happens with many Mollusca), or depositing them within the body of the female. It is curious that, in some of the lowest Fishes, we should return to one of the simplest conditions of this organ, a mass of vesicles, without any excretory duct. In these cases the secretion formed within the vesicles escapes, by their rupture, into the abdominal