Observe how transparent it is, and with what easy, undulating grace it swims about; yet this swimmer has no arms, no legs, no tail, no backbone to serve as a fulcrum to moving muscles: nay, it has no muscles to move with. ’Tis a creature of the most absolute abnegations: sans eyes, sans teeth, sans everything;—no, not sans everything, for as we look attentively we see certain currents produced in the liquid, and on applying a higher magnifying power we detect how these currents are produced. All over the surface of the Opalina there are delicate hairs, in incessant vibration: these are the cilia.[24] They lash the water, and the animal is propelled by their strokes, as a galley by its hundred oars. This is your first sight of that ciliary action of which you have so often read, and which you will henceforth find performing some important service in almost every animal you examine. Sometimes the cilia act as instruments of locomotion; sometimes as instruments of respiration, by continually renewing the current of water; sometimes as the means of drawing in food—for which purpose they surround the mouth, and by their incessant action produce a small whirlpool into which the food is sucked. An example of this is seen in the Vorticella (Fig. 2).
Having studied the action of these cilia in microscopic animals, you will be prepared to understand their office in your own organism. The lining membrane of your air-passages is covered with cilia; which may be observed by following the directions of Professor Sharpey, to whom science is indebted for a very exhaustive description of these organs. “To see them in motion, a portion of the ciliated mucous membrane may be taken from a recently-killed quadruped. The piece of membrane is to be folded with its free, or ciliated, surface outwards, placed on a slip of glass, with a little water or serum of blood, and covered with thin glass or mica. When it is now viewed with a power of 200 diameters, or upwards, a very obvious agitation will be perceived on the edge of the fold, and this appearance is caused by the moving cilia with which the surface of the membrane is covered. Being set close together, and moving simultaneously or in quick succession, the cilia, when in brisk action, give rise to the appearance of a bright transparent fringe along the fold of the membrane, agitated by such a rapid and incessant motion that the single threads which compose it cannot be perceived. The motion here meant is that of the cilia themselves; but they also set in motion the adjoining fluid, driving it along the ciliated surface, as is indicated by the agitation of any little particles that may accidentally float in it. The fact of the conveyance of fluids and other matters along the ciliated surface, as well as the direction in which they are impelled, may also be made manifest by immersing the membrane in fluid, and dropping on it some finely-pulverized substance (such as charcoal in fine powder), which will be slowly but steadily carried along in a constant and determinate direction.”[25]
It is an interesting fact, that while the direction in which the cilia propel fluids and particles is generally towards the interior of the organism, it is sometimes reversed; and, instead of beating the particles inwards, the cilia energetically beat them back, if they attempt to enter. Fatal results would ensue if this were not so. Our air-passages would no longer protect the lungs from particles of sand, coal-dust, and filings, flying about the atmosphere; on the contrary, the lashing hairs which cover the surface of these passages would catch up every particle, and drive it onwards into the lungs. Fortunately for us, the direction of the cilia is reversed, and they act as vigilant janitors, driving back all vagrant particles with a stern “No admittance—even on business!” In vain does the whirlwind dash a column of dust in our faces—in vain does the air, darkened with coal-dust, impetuously rush up the nostrils: the air is allowed to pass on, but the dust is inexorably driven back. Were it not so, how could miners, millers, iron-workers, and all the modern Tubal Cains contrive to live in their loaded atmospheres? In a week, their lungs would be choked up.
Perhaps, you will tell me that this is the case: that manufacturers of iron and steel are very subject to consumption; and that there is a peculiar discoloration of the lungs which has often been observed in coal miners, examined after death.
Not being a physician, and not intending to trouble you with medical questions, I must still place before you three considerations, which will show how untenable this notion is. First, although consumption may be frequent among the Sheffield workmen, the cause is not to be sought in their breathing filings, but in the sedentary and unwholesome confinement incidental to their occupation. Miners and coal-heavers are not troubled with consumption. Moreover, if the filings were the cause, all the artisans would suffer, when all breathe the same atmosphere. Secondly, while it is true that discoloured lungs have been observed in some miners, it has not been observed in all, or in many; whereas, it has been observed in men not miners, not exposed to any unusual amount of coal-dust. Thirdly, and most conclusively, experiment has shown that the coal-dust cannot penetrate to the lungs. Claude Bernard, the brilliant experimenter, tied a bladder, containing a quantity of powdered charcoal, to the muzzle of a rabbit. Whenever the animal breathed, the powder within the bladder was seen to be agitated. Except during feeding time, the bladder was kept constantly on, so that the animal breathed only this dusty air. If the powder could have escaped the vigilance of the cilia, and got into the lungs, this was a good occasion. But when the rabbit was killed and opened, many days afterwards, no powder whatever was found in the lungs, or bronchial tubes; several patches were collected about the nostrils and throat; but the cilia had acted as a strainer, keeping all particles from the air-tubes.
The swimming apparatus of the Opalina has led us far away from the little animal, who has been feeding while we have been lecturing. At the mention of feeding, you naturally look for the food that is eaten, the mouth and stomach that eat. But I hinted just now that this ethereal creature dispenses with a stomach, as too gross for its nature; and of course, by a similar refinement, dispenses with a mouth. Indeed, it has no organs whatever, except the cilia just spoken of. The same is true of several of the Infusoria; for you must know that naturalists no longer recognize the complex organization which Ehrenberg fancied he had detected in these microscopic beings. If it pains you to relinquish the piquant notion of a microscopic animalcule having a structure equal in complexity to that of the elephant, there will be ample compensation in the notion which replaces it, the notion of an ascending series of animal organisms, rising from the structureless amœba to the complex frame of a mammal. On a future occasion we shall see that, great as Ehrenberg’s services have been, his interpretations of what he saw have one by one been replaced by truer notions. His immense class of Infusoria has been, and is constantly being, diminished; many of his animals turn out to be plants; many of them embryos of worms; and some of them belong to the same divisions of the animal kingdom as the oyster and the shrimp: that is to say, they range with the Molluscs and Crustaceans. In these, of course, there is a complex organization; but in the Infusoria, as now understood, the organization is extremely simple. No one now believes the clear spaces visible in their substance to be stomachs, as Ehrenberg believed; and the idea of the Polygastrica, or many-stomached Infusoria, is abandoned. No one believes the coloured specs to be eyes; because, not to mention the difficulty of conceiving eyes where there is no nervous system, it has been found that even the spores of some plants have these coloured specs; and they are assuredly not eyes. If, then, we exclude the highly-organized Rotifera, or “Wheel Animalcules,” which are genuine Crustacea, we may say that all Infusoria, whether they be the young of worms or not, are of very simple organization.
And this leads us to consider what biologists mean by an organ: it is a particular portion of the body set apart for the performance of some particular function. The whole process of development is this setting apart for special purposes. The starting-point of Life is a single cell—that is to say, a microscopic sac, filled with liquid and granules, and having within it a nucleus, or smaller sac. Paley has somewhere remarked, that in the early stages, there is no difference discernible between a frog and a philosopher. It is very true; truer than he conceived. In the earliest stage of all, both the Batrachian and the Philosopher are nothing but single cells; although the one cell will develop into an Aristotle or a Newton, and the other will get no higher than the cold, damp, croaking animal which boys will pelt, anatomists dissect, and Frenchmen eat. From the starting-point of a single cell, this is the course taken: the cell divides itself into two, the two become four, the four eight, and so on, till a mass of cells is formed, not unlike the shape of a mulberry. This mulberry-mass then becomes a sac, with double envelopes, or walls: the inner wall, turned towards the yelk, or food, becomes the assimilating surface for the whole; the outer wall, turned towards the surrounding medium, becomes the surface which is to bring frog and philosopher into contact and relation with the external world—the Non-Ego, as the philosopher, in after life, will call it. Here we perceive the first grand “setting apart,” or differentiation, has taken place: the embryo having an assimilating surface, which has little to do with the external world; and a sensitive, contractile surface, which has little to do with the preparation and transport of food. The embryo is no longer a mass of similar cells; it is already become dissimilar, different, as respects its inner and outer envelope. But these envelopes are at present uniform; one part of each is exactly like the rest. Let us, therefore, follow the history of Development, and we shall find that the inner wall gradually becomes unlike itself in various parts; and that certain organs, constituting a very complex apparatus of Digestion, Secretion, and Excretion, are all one by one wrought out of it, by a series of metamorphoses, or differentiations. The inner wall thus passes from a simple assimilating surface to a complex apparatus serving the functions of vegetative life.
Now glance at the outer wall: from it also various organs have gradually been wrought: it has developed into muscles, nerves, bones, organs of sense, and brain: all these from a simple homogeneous membrane!
With this bird’s-eye view of the course of Development, you will be able to appreciate the grand law first clearly enunciated by Goethe and Von Baer, as the law of animal life, namely, that Development is always from the general to the special, from the simple to the complex, from the homogeneous to the heterogeneous; and this by a gradual series of differentiations.[26] Or to put it into the music of our deeply meditative Tennyson:—