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from a germ, draws together the nutrient materials supplied by the surrounding air and moisture, combines them into new products, appropriates these products to itself by incorporating them with its own structure, multiplies its kind when it has itself attained maturity, and finally dies after performing all the functions of the most highly organized plants. And yet this cell is but a solitary sacculus, or minute bag, whose wall is composed of a transparent colourless membrane, whilst its cavity contains a fluid, in which are held a number of granular particles of a red or green hue, mingled with others that consist of starch. Of aggregations of such cells, each of them a distinct individual, the 'red snow,' the 'gory dew,' the
green matter of Priestley,' and various other low forms of vegetation developing themselves in damp situations, are chiefly composed ;—the cells being generally united by a gelatinous substance in which they are imbedded—but each one living for and by itself alone. Now, even in these insignificant tribes, we have the two modes of reproduction, as possessed by the higher plants, clearly sketched out. Whilst the cells continually multiply themselves by a system of subdivision, in which each splits as it were into two, an entirely new generation arises from the approximation of two cells, and the mingling of their contents. The immediate result is the formation of a sporangium, from which the new family is to spring, as the higher plant does from its seed. Now the process of multiplication,
. by the mere doubling of cells, is obviously analogous to the multiplication of the flowering plant by leaf-buds; the same that, in fact, takes place whenever there is a new growth in continuity with the old. But the act of conjugation, and the formation of the sporangium, represent the essence both of the flowering of the higher plants and of the ripening of the ovule. The fertilization of this consists (as we shall hereafter see) in the mixture of the contents of the two cells which are set apart for the purpose, and which differ from the conjugating cells of the lowest Cryptogamia in no particulars of importance.
The process of conjugation has long been known to take place in a certain genus of Confervæ, which received the name of Zygnema in consequence. But the phenomenon was considered exceptional until within a very recent
period. It has lately, however, been recognised among so many other tribes of the lower Algæ, that it must henceforth be looked upon rather as the rule. The fact that the origination of new generations proceeds from conjugation, or from the union of the cell-contents of two parent individuals, as well in the lowest Cryptogamia as in the highest flowering-plant, has obviously a most important physiological bearing—for it leads us to revert to the opinions of those naturalists who have maintained the existence of sexes in the Cryptogamia generally. At the same time it affords a valuable means of discrimination in regard to such ambiguous forms as the Desmidee and Diatomeæ, whose animal or vegetable nature may be probably determined by this test more satisfactorily than by any other; nothing that truly resembles the conjugation of plants having been yet seen in any tribe of an undoubtedly animal character.
The careful study of these simple forms of vegetation has also led to the discovery that active movements, strongly resembling those of many animalcules, are performed by bodies whose vegetable character is undeniably proved by the nature of the structures into which they are subsequently developed; in which fact we have the explanation of the strange doctrine of former microscopists, who taught that there are beings which are animals in the earlier stage of their existence, and plants in the later. These movements depend on the same agency as those of many lower animals, namely, on the vibration of little hair-like filaments, termed cilia, from their resemblance to eyelashes. The cilia first strike the water with a broad surface, like that of an oar; whilst, in returning to their original position, they present a feather-edge to the liquid, and thus propel with great energy the body to which they are attached. This ciliary movement however is not confined to animalcules. By means of it the oyster, though it does not itself move, yet creates a current in the surrounding water-bringing food to its mouth, and oxygen to its gills. In higher animals it covers many of the membranous surfaces, especially those of the respiratory organs. Even in man there are incalculable numbers of such cilia constantly at work in his air-passages, which prevent the accumulation of the mucous secretions in the smaller tubes, and assist in carrying them away. This addition to our knowledge of ciliary action has been chiefly the result of the systematic observations of Professors Purkinje, Valentin, and Sharpey, and was one of the earliest and most valuable fruits of the achromatization of the microscope.
That the red colour of the blood of the higher animals does not exist in the fluid, but only in certain particles which float in its current, was known to Leeuwenhoek, and perhaps to still older microscopists : and the forms of these particles were observed to vary in different classes of animals. It was not known until recently, however, that these floating particles are true cells, analogous to those of the simplest Algæ-each having an independent life of its own, whilst all are subservient to the life of the being through whose vessels they are carried-though in what precise way they are so has not yet been satisfactorily ascertained. It has been further proved by the researches of Mr. Gulliver, that there is not only a difference of form, but also of size, between the blood-discs of various animals; so that it is in many instances possible to distinguish with certainty between the blood of two animals belonging even to the same natural group. The contrast in size is very strongly marked in particular cases. While the diameter of the circular Human blood-disc averages something less than one three-thousandth part of an inch, that of the circular blood-disc of the Musk-deer is no more than one twelve-thousandth; and the large oval blood-disc of the Proteus measures nearly as much as one three-hundredth of an inch in length, and half that amount across its short diameter. It is a very remarkable fact that all the animals yet known, which agree with the Proteus in the retention of the gills after the lungs are developed, -remaining, in fact, all their lives in the condition of great tadpoles,agree with it also in possessing blood-discs of unusual size. The fact that this character is presented by the blood-discs of the Lepidosiren, has been regarded as of no mean importance in discussing the real affinities of that curious reptile-like fish, or fish-like reptile.
We shall afterwards advert to the microscopic analysis recently made of those various soft tissues of animals and plants, which are the immediate instruments of their vital operations, and on whose nature any general expression of
the phenomena of life must be founded. But it is more convenient to consider previously the structure of the hard parts of-animals, such as bones, teeth, shell, &c. ; which present features of great interest, not merely to the physiologist who concerns himself about the vital phenomena of beings now alive, but also to the geologist who seeks in the fragmentary remains of past generations for materials to aid him in constructing a history of the earth. . Of the organization of bone, nothing more can be made out by the naked eye, or through the help of ordinary magnifiers, than that the solid substance of its densest portions is traversed by a series of canals, which carry inwards the blood-vessels distributed over the investing membrane (the periosteum); and that the spongy texture, found in the ends of the long bones and between the superficial layers of the thicker flat bones, derives its character from the presence of a vast number of minute chambers, or cancelli, which are separated from each other by irregular and incomplete partitions, and into which also the nutritious vessels are prolonged. Nothing whatever was known of the structure of the bony matter itself; and no one could have even ventured to imagine the elaborate minuteness of its organization. But when a thin section even of the very densest bone is examined by a microscope of sufficient power, the solid substance is seen to be penetrated by innumerable tubuli of extreme minuteness, radiating from a series of isolated excavations, of tolerably regular shape and size, and so interlacing each other as to open a communication among all the neighbouring cavities. To compare small things with great, let our readers conceive themselves in a cavern excavated in the centre of a vast mountain, with no principal entrance, but hundreds of narrow passages piercing its solid walls. Whichsoever of these they follow, it will conduct them, after many windings, into a cavern similar to that which they had left; and after passing through a long series of such passages and caverns, they will at last emerge into open day. Such, on a scale almost inconceivably minute, is the structure of solid bone. Let us reduce the spacious cavern to an excavation of 1:1500th of an inch in length, and 1:5000th of an inch in breadth ; and let the passages be contracted from dimensions sufficient to admit of our entrance to a diameter of 1.20,000th of an inch (which is further diminished in the smallest branches to 1.60,000th of an inch), and we then shall have some idea of the wonderful contrivance with which this solid and apparently impervious substance is channelled out, like a coal-field by the miner. There cannot be a doubt but that this curious organization is in some way subservient to the nutrition of bone. The tubuli are far too minute to allow of the entrance of the blood as a whole ; its floating particles, indeed, being nearly as large as the caverns themselves. But the great regularity of their arrangement, which is always adapted to keep up a communication between the interior of the bone and the nearest point to which blood is conveyed, seems to justify the inference that they take up from the circulating current, and distribute through the texture, such portions of the fluid as the bones require for their maintenance and consolidation. Very thin bones are nourished by blood, which is simply distributed over their surface; but larger and thicker bones are traversed by passages, into which the vessels penetrate.
We are too conscious of the difficulty of conveying by words alone a distinct idea of so complex a structure, to venture to be sure that we have carried with us every one of our readers through the preceding description. We trust, however, that we have fully impressed them with the wonderful elaborateness displayed in the minute structure of bone. This elaborateness has no parallel in any of the hard tissues of which the skeletons of invertebrated animals are composed. And it seems designed, on the one hand, to minister to the continual changes which bone must undergo during the period of its growth, and, on the other, to confer upon bone that power of self-reparation which it so remarkably displays after disease or injury. It is interesting to observe that whilst, as in the case of the blood-corpuscles, there is no relation whatever between the size of the animal and the dimensions of the elementary parts of its bony skeleton,-the length of the cavernules and the diameter of the tubuli being nearly the same in the Elephant and in the Mouse, in the gigantic extinct Iguanodon and in the smallest existing Lizard, there is yet a marked difference in these particulars between ani