The Metazoa, Or The Multicellular Forms Of Life

( Originally Published 1909 )

UNLIKE the Unicellular department of life, whose chief characteristic is fixity, that of the Metazoa is development. This development is not alone into a great variety of forms, but much more into a progressive elevation in function, or in the kind of work which is performed. Thus connective tissue cells, whose working is purely mechanical, are inferior to muscle cells, whose chief function is to pull and then to relax, for the relations of muscles to animal heat do not concern us now.

But this muscle function of contraction and relaxation is simple compared with the functions of a collection of nerve cells.

As function, however, is a gauge of life, we may say that muscle cells are much less alive than the nerve cells in the spinal cord, and these in turn much less alive than nerve cells in the brain.

But what is meant by the term, alive? It is altogether too great a word to be defined. An insect is alive, and a stone is not. A little spider can make a web, which nothing however great if it is not alive can do. Only that which is alive can " make " anything. Hence in the metazoa one form is more alive than the other below it, and so on through the whole series from a simple living growth to living movement, and then on to sensation, thought, and purpose.

But this development of the metazoa was very slow in its course, the slowest, greatest, and last of all being nervous development. We might almost liken it to a prolonged course of education, each succeeding class learning what its predecessor had, and then adding to it.

It is this great feature of progressive development which has led biologists, from Darwin down, to restrict their attention to the metazoa. As we have already discussed the various theories propounded as explanatory of the course of development, we need not recapitulate them here. But a great and hitherto insuperable difficulty inexorably and equally attends all these speculators from the very start. If they had only full-grown plants and animals to deal with, the task would be comparatively easy. But, as we have seen, each metazoan form of life begins as a single microscopic cell, which is then more itself and nothing but itself than it will ever be afterward. In other words it looks at first altogether like one of those eternal unicellular things which never change nor develop for millions of years. But that which makes the vast difference between this metazoan unicellular thing and the old unicellular things must all depend on the internal' organization of its microscopic body. There is no way to get round that fact. The whole hereditary past and the whole developmental future, down to its smallest details, are together locked up in that minute speck of living matter.

The imagination itself refuses to picture at this stage what the internal organization of this wonderful little dot must be, and asks to wait till its subsequent behavior can be observed. Accordingly, the first thing noted about this metazoic cell is that instead of reproducing its like in all particulars as formerly, it divides into two cells, and these then into four, and these into eight, and these again into sixteen, and so on into an indefinite number of divisions.

But now comes the greatest of all the mysteries of physical life, to which is due the difference between the old order of things and the new. Multiplication of cells, all with fixed hereditary and specific properties, was a great feature of the old order, but each cell was then an independent and finished living thing. In the metazoa, on the other hand, not independence but interdependence of cells is the absolute law. Their cells are never free unless they are being cast out of the community. A mere indefinite multiplication of their cells, in which they are said to proliferate indiscriminately, or even a sign of weakening of their reciprocal interdependence, signifies disease and death. All their cells instead are held together by a wonderful something which " organizes " their development along very definite lines. Thus in the higher animals after the dividing but adhering cells have come to resemble a mulberry in appearance, a division into three layers soon appears. From the first layer develop in time, by various infoldings, the nervous and some skin tissues ; from the second the connective tissue, the muscles, blood-vessels, and bones; and from the third the lining of the alimentary canal and its associated viscera, etc. If the further steps of development of organs out of these primary divisions had not been carefully followed, no one would dream what the word " differentiation " used by biologists to denote the process, means. It means that each cell, as growth goes on, takes its place where it ought to, in other words along predetermined lines. Every one of the many and diverse tissues, for example, which make up an eye, fit their places as parts of an eye and not of an ear. And so of the component parts of all the other organs. Fitness local, fitness general, fitness universal meets us at every turn, and so sure is this interdependence and interrelation of parts that an anatomist can often reconstruct an animal from a single tooth.

All these internal adjustments present at last an infinitely more complex and complete mechanism than the watch which Paley instanced as an illustration of intelligent design in the making. Paley's argument, however, in our times has been quite displaced by the great conception of mechanical development, or as it is called, Evolution, a conception which while true enough in principle is so far wholly unprovable in details. Evolution through descent of the later from earlier forms is believed in by all biologists, however diverse their opinions about what the process of evolution itself was. But here comes the great difficulty, namely, that general descent does not afford the least insight into individual descent. The real problem of life, therefore, is how the microscopic single cell, with which each individual among the metazoa begins, should virtually contain not only all the structural peculiarities of the body of its parent with-out variation from its hereditary pattern, but should also have the power to deter-mine where the untold millions of cells to grow from it are to find their proper places in the future adult body. What combination of physics and chemistry could pro-duce this thing, when the inconceivably complex internal make-up of such a microcosm as that single primordial cell can-not be conceived of even by metaphysicians?

But something by way of explanation had to be attempted, and therefore various theories have been propounded. Now theories are as indispensable in scientific construction as scaffolding is in the erection of a great building. The mistake, then, would be to lay the solid facts we get on the scaffolding instead of on the previous settled layer of stones, a mistake which is not uncommon. Hence Darwin, when he found himself confronted by this special problem, ten years after he published his book on the Origin of Species, propounded a theory to which he gave the name of Pangenesis.

This pangenesis postulated a well-nigh infinite number of small gemmules, which are given off by every cell in the body and which make their way to the primal microscopic dot, so that this contains a representation of every part of the parent body. But as a man who had lost his right leg would have no right leg gemmules to send thereafter, so a child born to him after his accident would be minus that leg. This and similar considerations led Darwin finally, with characteristic candor, to pronounce this theory " unfortunate."

Since Darwin, or at least for thirty years past, eminent biologists such as Roux, Weismann, Hertwig, De Vries, Driesch, Boveri, Wilson, and a host of others, have been observing, experimenting, and theorizing, without coming to the least agreement on these subjects ; many of them virtually reverting to Darwin's pangenesis, with the result as expressed by Prof. Wilson * (p. 433), " The truth is that an explanation of development is at present beyond our reach."

We would fain emerge, therefore, from the shapeless fogs of the realm of theory into quite another territory whose chief outlines are plainly discernible and which are these. That the old unicellular forms, which still hold the greater portion of the field, seem, most ominously for us, to have resented the appearance of the multicellular forms on this globe and have been waging a ceaseless deadly war against them ever since. We have already seen that so soon as a tree or large animal dies, its lifeless body is at once attacked by unicellular forms in a most effective fashion, as if they would get rid of every trace of such multicellular things. But their activities are not only post-mortem. Instead the unicellular forms remain ever on the watch to break through every barrier raised by the metazoa for self-defence. The chief barrier is the thin basement membrane, as it is called, on which grow the cells of the external skin and of the internal skin or mucous membrane. So long as these covering layers of cells are healthy, billions of the enemy may accumulate there without effecting anything. The instructed surgeon, however, well knows that he cannot make the smallest incision or sometimes even puncture of this protective envelope without peril to his patient from the inrush of unicellular invaders, and the great triumphs of mod-ern antiseptic surgery are due to the practical application of the great principle that multicellular must be guarded at every point from unicellular life. In medicine the same principles are equally important, because if any part of the protecting envelope is damaged by inflammation or injury, straightway an entrance for the deadly old enemies is established.

But a second and important line of de-fence is found in the inherent power of the healthy bodily cells to resist the invaders. This is well illustrated by an experiment in which after minute injuries have been inflicted by long, fine, but sterilized needles in the liver, spleen, kidneys, and limbs of rabbits, and then a virulent culture of pus-forming bacteria is injected into a vein of the rabbit's ear; these bacteria then circulate with the blood over the whole body without affecting any of its cells except just where the cells have been injured by the needles; there they at once begin to form abscesses. The uninjured cells are then said to be endowed with a vital resistance to the invaders, which is wanting in those cells which have been devitalized by injury. A third line of de-fence is found in the healthy blood itself, which contains well-identified ingredients called opsonins, which enable the white corpuscles of the blood to attack the invading bacteria and eat them up.

But what is this all-important vital resistance? We need not say how important this is when, if a colony of tubercle bacilli finds a lodgment in a man's lung, his prospects will then depend on his local or general stock of vitality. The lower this is the sooner his funeral.

Meantime we have been reading the assertions of a school of biologists, that there is no such thing as vitality or vital force. The trained physician, feels then like bidding these mere theorists to hold their peace, for the principles of his great science of Hygiene are as well settled as those of any science whatever. Those principles, in short, are to help life fight life, that is, so to promote, on the one hand, the vitality of the individual by every means possible, that he can success-fully resist invading micro-organisms, and, on the other hand, to make his victory easier by weakening the vitality of the unicellular enemies. Vitality, therefore, is doubly enlisted. The physician strives to increase the general health of the individual by good food, fresh air, sunlight, cleanliness or washing, and exercise. Meantime if he enters the dark, close rooms of a crowded tenement, he knows that pernicious micro-organisms are there in countless millions. But fresh air makes most of them sick, light also enfeebles them or kills many of them outright, cleanliness, for the same reason, he elevates into a religious duty, and he is very particular in his inquiries about foods, especially milk, which he insists must be sterilized, or in other words have the bacteria in it destroyed. Chemistry and physics he never thinks of except as adjuncts in a purely vital war.

In practical medicine this subject of vitality never leaves us. Serious diseases are not always due to micro-organisms. Different 'physical causes may lead, for example, to progressive heart and kidney disease, in which the physician vainly tries to arrest the downward course. Finally things take a turn which has led medical authorities to make the seemingly paradoxical statement that but few people die of their diseases. It is because the slow, devitalizing effects of previous disease on the body cells, especially in chronic heart and kidney affections, at last open the way for a host of micro-organisms to enter unopposed, and the post-mortem table then reveals how the actual cause of death came from a vast invasion of what are called the terminal infections. The survival of the fittest among metazoa therefore belongs to those who have the most vitality.

Solely to illustrate how the importance of these principles cannot be over-estimated in their practical applications, I will relate a recent personal experience. On January 26, 1909, I was called in consultation to see a little girl five years of age, who six days before began with a severe attack of scarlet fever. On the third day of this disease malignant diphtheria also supervened, and a thick membrane with a gangrenous odor covered all the visible surfaces of the throat, while the glands of the neck were greatly swollen. I told the attending physician that the child would not probably die from her two infections, but from the general invasion through her ulcerated throat of numberless streptococci. As the child could not swallow, she was to be treated without any medicine, for the hypodermic injections of strychnine for stimulating the failing heart which the physician had been giving would be no more efficacious against the condition than if the little syringe was emptied into a fire. The only recourse left was to freely douche the throat as one would wash a dirty sidewalk with a hose. We might thus wash away the armies of micro-organisms and stop the further absorption of the diphtheria toxin from the decomposing membrane, while the patient's vital powers might be able to overcome the organisms which had already gained entrance, provided that no further additions were allowed. With a rubber ball between her teeth to keep her mouth wide open, a couple of gallons of hot water with chlorate of potash and peppermint were poured into the mouth every two hours night and day, from an elevated fountain syringe: for if the mouth be kept open nothing is swallowed, but the current strikes against the back of the throat and returns by the tonsils, washing everything before it as it pours out of the mouth. I saw her again on the 28th, and was gratified to note a favorable change from the deathly expression of her face at the first visit. She still could not swallow, but the glands in the neck had begun to diminish in size, though much membrane remained in the throat. On the 30th I found that she could swallow, but that there remained the following complications to expect, like so many rocks in shooting rapids with a canoe. First the scarlatinal agent would invade the ears ; then twenty-five per cent. of deaths from scarlet fever are due to pneumonia; then pleurisy is especially to be dreaded in these cases, as it so often ends in the abscess of the pleura called empyema : then death might be caused by the diphtheria toxin dissolving, as it were, strands of the heart muscle, and lastly streptococcic invasion of the kidneys might end the scene. All of these dangers occurred to the child in their order except the last. Abscesses in both ears developed the next day, but a skilful aurist was at hand and freed the ears completely. This, of itself, was no small gain, for many cases of hard hearing throughout life date from the ear inflammations by scarlet fever in childhood, and I have been told that the same cause accounts for twenty per cent. of the deaf mutes in our asylums. On February 2, I found a patch of pneumonia with pleurisy in the left lung between the shoulder-blades, but in five days the child's vital powers dealt successfully with them both. She was now quite hungry and always had her doll with her. Lastly, the weak intermittent pulse showed that the diphtheria poison was doing its work. Now on the twenty-fifth day of her illness for the first time she took medicine, but it took longer to relieve this complication than in the case of any of the others, before she finally recovered completely.

I have told this long story simply to show what a great and real thing vitality is. The physician's duty is to see to it that vitality has fair play in such a battle for life.

Therefore, instead of regarding the principles of Hygiene as one would general dictates to be good and virtuous, these truths furnish quite specific reasons for attending to health, for only thus can we postpone the inevitable end. Because the supreme fact is that not by physical causes is our stay on earth usually ended. Physical causes may occasionally terminate life by storm, earthquakes or flood, or through human perversity by wars, but aside from such happenings it is by the agency of a great living kingdom that our mortal bodies return dust to dust.

It is also life which destroys life when death is caused by those dread and well-named malignant diseases, cancer and sarcoma. Never before in medical history has there been such diligent and intelligent search for the origin of these terrible affections as now, until it begins to look as if instead of coming from the old class of unicellular micro-organisms they come from a native metazoan cell which has deserted to the enemy. As with other renegades its greatest hostility is to its old associates, and it is all the more dangerous because it retains some of the metazoan powers.

We can explain this in a few words. We have said that the great wonder of a metazoan body is the interdependence of its cells, each having and each keeping its own place. Moreover, though each metazoan cell retains its original endowment of indefinite multiplication, yet it always holds this in check in deference to the rights of its neighbors. If by any chance a normal metazoan cell be freed from neighbors, then it grows by the mil-lion till it comes to neighbors again, where-upon it resumes its proper consideration for their territorial rights. This is well illustrated when the surgeon implants on a large ulcerated surface on the skin which will not heal, minute pieces of normal skin grafts whose cells then multiply actively, far more than in their original place, with the significant addition that they now throw out invisible germinal particles to-wards the nearest healthy cells at the edge of the sore, stimulating them to aid in forming a bridge across to the grafts. In time the open sore is thus covered with good skin, whose cells at once quit multiplying so soon as they join healthy skin cells.

But now among apparently the most well behaved metazoic cells there appears, no one yet knows why or how, a pure rebel against life's beneficent law of mutual dependence and co-operation. This rebel will grow and multiply just where it pleases without the least consideration for the traditional claims of others. Thus a cell from the mucous membrane of the lower intestine leaves that place and finds its way by the blood stream till it lodges in the brain, and then grows where it has no more business to be than a chimney sweep has to be ensconced in a lady's boudoir.

It not only pushes aside its neighbors, but invades their tissues and by intruding between nerves causes agonizing pains. Meantime it retains enough of its metazoan powers to give rise to organized growths or tumors with blood-vessels and other tissues, but with such poor vitality that these growths ultimately break down into repulsive and highly poisonous ulcers. Unlike the old unicellular enemies, it cares little for hygiene, for it counts emperors and queens among its victims as well as laborers and washerwomen, destroying those also who live moral lives just the same as those who do not. That its origin lies in the deepest processes of cell nutrition is shown, as we have already stated, by its occurrence in all vertebrates, not excepting fishes, for trout die of cancer as well as we ourselves.