the Poppy (Fig. 190, A) presents a regular reticulation upon its

surface, pits, for the most part hexagonal, being left between projecting walls; that of Caryophyllum (D) is regularly covered with curiously jagged divisions, every one of which has a small bright black hemispherical knob in its middle; that of Amaranthus hypochondriacus has its surface traced with extremely delicate markings (B); that of Antirrhinum (?) is strangely irregular in shape (c), and looks almost like a piece of furnace-slag; and that of Bignonia (E) is remarkable for the beautiful structure of the translucent membrane which

sur

rounds it, the radiating lines shown in the figure being found under a higher magnifying power to consist of rows of elongated spiral cells. Such are seen, too, in the like delicate membrane that surrounds several other seeds, as those of Sphenogyne speciosa and Lophospermum erubescens, which, from possessing this appendage, are spoken of as "winged." The most remarkable development of this structure is said by Mr. Quekett to exist in a seed of Calosanthes Indica, an East Indian plant, in which the wing extends more than an inch on either side of the seed. Some seeds are distinguished by a peculiarity of form, which, although readily discernible by the naked eye, becomes much more striking when they are viewed under a very low magnifying power; this is the case, for example, with the seeds of the Carrot, whose long radiating processes make it bear, under the Microscope, no trifling resemblance to some kinds of star-fish; and with those of Cyanthus minor, which bear about the same degree of resemblance to shaving-brushes. In addition to the preceding, the following may be mentioned as seeds easily to be obtained, and as worth mounting for opaque objects:-Anagallis, Anethum graveolens, Antirrhinum, Begonia, Carum carui, Coriopsis tinctoria, Datura, Delphinium, Digitalis, Elatine, Erica, Gentiani, Gesnera, Hyoscyamus, Hypericum, Lepidium, Limnocharis, Linaria, Lychnis, Mesembryanthemum, Nicotiana, Orobanche, Petunia, Reseda, Saxifraga, Scrophularia, Sedum, Sempervivum, Silene, Stellaria, and Verbena. The following may be mounted as transparent objects in Canada balsam:-Drosera, Hydrangea, Mono

1

tropa, Orchis, Parnassia, Pyrola, Saxifraga. The seeds of Umbelliferous plants generally are remarkable for the peculiar vittæ, or receptacles for essential oil, which are found in their coats. Various points of interest respecting the structure of the teste or envelopes of seeds,-such as the fibre-cells of Cobra and Collomia, the stellate cells of the Star-Anise, and the densely consolidated tissue of the "shells" of the Coquilla-nut, Cocoa-nut, &c.,-having been already noticed, we cannot here stop to do more than advert to the peculiarity of the constitution of the "husk" of the Corngrains. In these, as in other Grasses, the ovary itself continues to envelope the seed, forming a covering to it, that surrounds its own testa; this covering (which forms the "bran" that is detached in grinding) is composed of hexagonal cells of remarkable regularity and density; and these are so little altered by a high temperature, as still to be readily distinguishable when the grain has been ground after roasting,-thus enabling the Microscopist to detect even a very small admixture of roasted Corn with Coffee or chicory, without the least difficulty.'

These lists have been chiefly derived from the "Micrographic Dictionary," p. 572. In a case in which the Author was called upon to make such an investigation, be found as many as thirty distinctly recognizable fragments of this cellular envelope, in a single grain of a mixture consisting of Chicory with only 5 per cent. of roasted Corn.

CHAPTER IX.

MICROSCOPIC FORMS OF ANIMAL LIFE:-PROTOZOA; ANIMALCULES.

259. Protozoa.-Passing on, now, to the Animal Kingdom, we begin by directing our attention to those minute and simple forms, which correspond, in the Animal series, with the Protophyta in the Vegetable (Chap. VI); and this is the more desirable, since the formation of a distinct group, to which the name of Protozoa (first proposed by Siebold) may be appropriately given, is not merely one of the most interesting results of recent Microscopic inquiry, but is a subject on which it is particularly important that the Microscopic observer should know what the Physiologist believes himself to have ascertained. This group, which must be placed at the very base of the animal scale, beneath the great subkingdoms marked out by Cuvier, is characterized by the extreme simplicity that prevails in the structure of the beings composing it; these being either isolated cells, or aggregations of cells wherein no such differentiation of parts exhibits itself, as constitutes the "organs" of even the simplest Zoophyte or Worm. We have in the first place to consider, therefore, what are the essential characters of the Animal cell; and what are the precise relations of the Protozoa to the Protophyta, to which they seem to bear so close an affinity.

260. The Animal cell, in its most complete form, is comparable in most parts of its structure to that of the Plant; but differs from it in the entire absence of the "cellulose-wall," or of any thing that represents it, the cell contents being enclosed in only a single limitary membrane, the chemical composition of which (being albuminous) indicates its correspondence with the primor dial utricle (§ 147). In its young state, it seems always to contain a semi-fluid plasma, which is essentially the same as the "protoplasm" of the Plant, save that it does not include chlorophyll-granules; and this may either continue to occupy its cavity (which is the case in cells whose entire energy is directed to growth and multiplication), or may give place, either wholly or in part, to the special product which it may be the function of the cell to prepare. Like the Vegetable cell, that of Animals very commonly multiplies by duplicative subdivision; and it also (especially among Protozoa) may give origin to new cells,

by the breaking up of its contents into several particles; but new cells are not unfrequently to be met with, especially in the nutritive fluids of such Animals as possess a distinct circulation, which have not directly originated in either of these modes from a previously existing cell, but which have been developed by a process of free cell-formation, namely, by the aggregation of organic molecules, floating in these fluids, into little masses, of which the external particles coalesce into a membranous cellwall, whilst the interior liquefy into cell-contents. This can only take place, however, in a liquid which has undergone elaboration in the interior of a highly-organized living body; and we find no traces of such free cell-formation among the members of the group we are first to investigate.

261. As we have seen (§ 150) that, among the lowest Protophytes, the general attributes of a cell may exist in a minute mass of protoplasm which is not bounded by a limitary membrane, the differentiation between cell-wall and cell-contents not having yet manifested itself,-so, among the lowest Protozoa, we find the power of maintaining an independent existence to be possessed by similar particles of that peculiar blastema, or formative substance, to which the name of sarcode has been given by Dujardin (who first drew attention to its extraordinary endowments), and which may be considered as the basis, not merely of the entire organisms of Protozoa, but of a large part of that of higher animals. The properties of this may be most fully understood by the careful study of a creature, which is by no means unfrequently to be met with in fresh and stagnant waters, vegetable infusions, &c., and which, from the great variety of forms it assumes, has received the designation of Pro

FIG. 191.

Amaba princeps, in different forms, A, B, C.

teus. This name, however, having been assigned to an animal of far higher organization, that with which we are now concerned is properly known as the Amoeba (Fig. 191). It may be

described as a minute mass of "sarcode," presenting scarcely any evidence of distinct organization, even of the simplest kind; for, as in the lowest forms of Vegetable cell (§ 148), there is not even a complete differentiation between the cell-wall and the cell-contents; the former not being composed of a distinct membrane, though obviously possessing more consistence than the latter, which are semi-fluid. A contractile vesicle (or rather, perhaps, a "vacuole") may be observed in some part of the body, which pulsates at tolerably regular intervals; and "vacuoles" or clear spaces are seen, surrounding the alimentary particles which have been received into the midst of the jelly-like substance. However inert and shapeless this minute body may be when first noticed, its possession of vital activity is soon made apparent by the movements which it executes, and by the changes of form which it undergoes; these being, in fact, part of one and the same set of actions. For the shapeless mass puts forth one or more finger-like prolongations, which are simply extensions of its gelatinous substance in those particular directions; and a continuation of the same action, first distending the prolonga tion, and then (as it were) carrying the whole body into it, causes the entire mass to change its place. After a short time another prolongation is put forth, either in the same or in some different direction; and the body is again absorbed into it. These changes seem to be connected with a movement of the semi-fluid particles in the interior of the mass, of which a current may be observed to "set" in the direction wherein the protrusion is about to take place, before the surface shows any projection. When the creature, in the course of its progress, meets with a particle capable of affording it nutriment, its gelatinous body spreads itself over or around this, so as to envelope it completely; and the substance (sometimes animal, sometimes vegetable) thus taken into this extemporized stomach, undergoes a sort of digestion there, the nutrient material being extracted, and any indigesti ble part making its way to the surface and finally being (as it were) squeezed out. Of the mode of reproduction of Amoeba, nothing is yet known, save that it undergoes multiplication by self-division, very much in the manner of the Protophytes, and that portions separated from the jelly-like mass, either by cutting or tearing, can develope themselves into independent beings. Consequently, as we are quite in the dark respecting the sexual operation, which (as all analogy would lead us to believe) must take place at some period of its life, it cannot be said that we are acquainted with more than one phase of its existence; and it is quite possible that, after many repetitions of the process of multiplication by self-division, some entirely new form may present itself, of which the Amoeba is (as it were) the larva. The completion of the life-history of this curious creature, therefore, is a most worthy object of Microscopic inquiry; and its abundance in many situations should prevent this from being a matter of