Germ layer

“ I am also far from asserting that the germ-plasm which, as I hold, is transmitted as the basis of heredity from one generation to another, is absolutely unchangeable or totally uninfluenced by forces residing in the organism within which it is transformed into germ-cells. I am also compelled to admit it as conceivable that organisms may exert a modifying influence upon their germ-cells, and even that such a process is to a certain extent inevitable. The nutrition and growth of the individual must exercise some influence upon its germ-cells . ”

“ The later system of Pasteur is founded on this same general truth, as are also the methods of lessening the pathogenesis of germs by subjecting them to compressed oxygen, to graduated heating, to an altered chemical condition of the culture medium, to antisepsis, etc. For a time such weakened cultures often retain their lessened pathogenesis, even through a succession of cultures in a susceptible animal body, acting as if the germ were indeed a distinct species. ”

“ Whether it is a fact or not that somatic variations can induce corresponding variations in the germ-cells, [65] and thus be transmitted by heredity, it is certainly true that all heritages must come through the germ-cells. For this reason, it is clearly seen that so far as evolution is concerned the germ-cells are the factors of fundamental importance in organisms. Therefore, we may repeat that environment is the sum-total of the conditions of life that affect the germ-cells directly or indirectly. ”

“ This does not ignore that the chemical products or enzymes which are constructed by the vital activity of the microbes, will dissolve or transform organic matter, but in the absence of the microbe no such enzyme can reproduce nor multiply itself and its action must therefore be exactly limited by its amount. The living germ itself is therefore the one effective factor, by which the contagious disease may be maintained and propagated. In its turn the living germ can only come from a pre-existing living germ. ”

“ The great host of species which are found in water, milk, air, in our mouths or clinging to our skin, and which are almost omnipresent in Nature, are capable of growing well enough in ordinary lifeless organic foods, but just as soon as they succeed in finding entrance into living human tissue their growth is checked at once by these antiseptic agents which are poured upon them. Such bacteria are therefore not pathogenic germs, and not sources of trouble to human health. ”

“ This is sufficient to afford a fair sample of the germ content of the mixture, and at the same time is insufficient to exert any inhibitory action when transferred to the subculture medium.
e. Subculture Medium. Bile Salt Broth.—
A fluid medium is essential in order to obtain immediate dilution of the germicide carried over; at the same time it is advantageous to employ a selective medium which favours the growth of the test germ to the [Pg 484] exclusion of organisms likely to contaminate the preparation, and if possible one which affords characteristic cultural appearances. ”

“ For the climate might well influence the germ through the body. The body being unhealthy and degenerate, the germ-cells must, one may suppose, suffer too. The degenerate pup born in England might well owe its degeneracy to effects wrought upon the germinal cells. In other words, such a case would indicate some general influence of the environment (including the environing body) on the germ. It does not convince us that particular modifications of body-cells as such are transmitted under normal and healthy conditions. ”

“ In fact, a favorable breeding ground is perhaps not second in importance to the presence of the germ itself, as the number of individual microbes, up to a certain point, which gain access to the human body, is probably of much more importance than the kind of germ. ”

“ While most of the so-called germ diseases are caused by the bacteria which we have been studying in the previous chapters, there are some whose inciting cause is to be found among organisms belonging to other groups. Some of these are plants of a higher organization than bacteria, but others are undoubtedly microscopic animals. Their life habits are somewhat different from those of bacteria, and hence the course of the diseases is commonly different. ”

“ Thereby would be found a connexion between the modified parts of the body and the germ-cells, the only connexion in existence. It is to be assumed, according to this author, that only a qualitative change in the nutritive fluid of the germ-cells could produce an effect: a quantitative change would only cause increased or decreased nourishment of the entire germ cells. ”

“ Hence we may fairly attribute to the adult organism influences which determine the phyletic development of its descendants. For the germ-cells are contained in the organism, and the external influences which affect them are intimately connected with the state of the organism in which they lie hid. If it be well nourished, the germ-cells will have abundant nutriment; and, conversely, if it be weak and sickly, the germ-cells will be arrested in their growth. ”

“ He believes that organs in the adult, not necessarily by direct action, but by action from a distance, may alter the germ. Of this, there is no proof at present. Others have suggested that just as the ductless glands pour into the blood chemical substances which materially affect the growth and development of other portions of the body, so similar enzymes, or other chemical substances, may be sent into the blood, which subsequently bathes the germ cells of the coming generation and produces the change. ”

“ How Disease Germs Grow.—Experiments having shown that no life is known to spring from inanimate matter, we may reasonably suppose that just as wheat does not grow except from seed, so no disease occurs without some disease germ to produce it. Then, again, we may logically assume that each disease is due to the development of a particular kind of germ. If we plant smallpox germs, we do not reap a crop of scarlatina or measles; but, just as wheat springs from wheat, each disease has its own distinctive germs. ”

“ From the view-point of cell-differentiation, which seems to me the most tenable position, there does not seem any evidence for, or any probability of, the occurrence of any roundabout mode of development of the germinal cells which could enable them to pick up acquired characters en route.
(d) We come now to the contention that the organism, being one and continuous, if any member suffers, the germ suffers with it. The organs of the body are not isolated or insulated; the blood is a common medium; the nerves ramify everywhere ”

“ It is therefore possible to conceive that at some time or other there has existed only one kind of germ, that variations from this one type have arisen in consequence of the modification wrought upon different individuals by their chance falling upon this or that degenerate tissue. Variation must lead to specialization, and finally we find diseases all dependent upon the action of germs, as different from one another as one species of animal differs from another. ”

“ But the everlasting wonder is, how all the characteristics of a species can be embodied in a germ so as to be reproduced by growth; and still more amazing is it how the nicer shades of organic modification are also transmitted by the germ so as to become hereditary. In some way the systems of organic molecules must be capable of taking and retaining an infinite number of inconceivably delicate impressions; for with the myriad forms of life there must be formed myriads of molecular modifications of germ structure. ”

“ It will be noticed that the next step in the food cycle is taken by the decomposition bacteria. These organisms, existing, as we have already seen, in the air, in the soil, in the water, and always ready to seize hold of any organic substance that may furnish them with food, feed upon the products of animal life, whether they are such products as muscle tissue, or fat, or sugar, or whether they are the excreted products of animal life, such as urea, and produce therein the chemical decomposition changes already noticed. ”

“ Methods of fighting Germ Diseases.—As we have seen, diseases produced by bacteria may be caused by the bacteria being directly transferred from one person to another, or the disease may obtain a foothold in the body from food, water, or by taking them into the blood through a cut or a wound or a body opening.
It is evident that as individuals we may each do something to prevent the spread of germ diseases, especially in our homes. We may keep our bodies, especially our hands and faces, clean. ”

“ Even a very minute trace of a specific germ-plasm possesses the definite tendency to build up a certain organism, and will develop this tendency as soon as the nutrition is, for some reason, favoured above that of the other kinds of germ-plasm present in the nucleus."
In place, then, of the direct continuity of germ-cells as distinct from body-cells, we have here the direct continuity of germ-plasm as opposed to body-plasm. The germ-plasm can give rise to body-plasm to any extent; the body-plasm can never give rise to germ-plasm. ”

“ The following statements comprise the most important facts known in regard to it, and its relation to the disease in man.
Undoubtedly the malarial germ has some home outside the human body, but it is not yet very definitely known what this external home is; nor do we know from what source the human parasite is derived. It appears probable that water serves in some cases as its means of transference to man, and air in other cases. From some external source it gains access to man and finds its way into the blood. ”

“ And so far from the fully developed organism’s being simply the germ plus the nutriment, which it has absorbed, it is probable that the adult contains neither in form, nor in substance, more than an inappreciable fraction of the constituents of the germ, and that it is almost wholly made up of assimilated and metamorphosed nutriment.”
This being true, it cannot be said that the germ (fertilized ovum) ever develops into a man or woman. On the contrary it is annihilated; and its identity is wholly lost among the daughter-cells which are made of the mother’s food. ”

“ In explanation of the phenomena of heredity he introduced a new theory, the "theory of the continuity of the germ-plasm." According to him the living substance in all organisms consists of two quite distinct kinds of plasm, somatic and germinal. The permanent germ-plasm, or the active substance of the two germ-cells (egg-cell and sperm-cell) , passes unchanged through a series of generations, and is not affected by environmental influences. The environment modifies only the soma-plasm, the organs and tissues of the body. ”

“ In order to determine whether or no the test germs have been destroyed, observations must always be continued—when growth appears to be absent—up to the end of seven days before recording "no growth."
h. Identification of the Organisms Developing in the Subcultivations after Contact in the Germ + Germicide Solution.—
This is based on the naked eye characters of the growth in the bile salt broth, supplemented where [Pg 485] necessary by plating methods, further subcultivations upon carbohydrate media and agglutination experiments. ”

“ It is also accounted for by the germicidal properties of the blood serum.
The resistance of the tissues may in some cases be due to the absence from them of some particular element necessary to the growth of a particular microorganism. This refractoriness varies in every species of animal in its relation to every form of germ. Different individuals of one species also vary in their susceptibility, and even different parts of the body vary in the same individual. The lower animals offer a greater resistance to pyogenic bacteria than do human beings. ”

“ I may remind the reader that many of the embalmed bodies of the sacred Egyptian animals must be four thousand years old, and that the species are identical with those now existing in the same locality. Now, since the quantity of germ-plasm contained in a single germ-cell must be very minute, and since only a very small fraction can remain unchanged when the germ-cell developes into an organism, it follows that an enormous growth of this small fraction must take place in every individual, for it must be remembered that each individual produces thousands of germ-cells. ”

“ There are, however, some species which cause mankind much trouble by interfering in one way or another with the normal processes of life. These pathogenic bacteria, or disease germs, do not all act alike, but bring about injury to man in a number of different ways. We may recognise two different classes among them, which, however, we shall see are connected by intermediate types. These two classes are, first, the pathogenic bacteria, which are not strictly parasitic but live free in Nature; and, second, those which live as true parasites in the bodies of man or other animals. ”

“ In the ground, the life of the bacteria is longer, and while experiments do not agree very well as to the exact length of time that the germ may live there, there seems to be evidence that they may live several months, if not a year or more. ”

“ Not only do certain known poisons circulating in the blood, or other fluid that may bathe the germ-cells of living creatures, profoundly affect the germ-cells, but many other substances probably have great influence upon them. ”

“ They do not, however, occur in the tissues of a healthy individual, either in the blood, muscle, gland, or any other organ. Secretions, such as milk, urine, etc., always contain them, however, since the bacteria do exist in the ducts of the glands which conduct the secretions to the exterior, and thus, while the bacteria are never in the healthy gland itself, they always succeed in contaminating the secretion as it passes to the exterior. Not only higher animals, but the lower animals also have their bodies more or less covered with bacteria. ”