This suggests possibly a specific action of the enzyme. If this slight reversible action had any biological significance (which might be possible, since in the organism secondary favourable condi­tions might be at work which are lacking in vitro) there should be a parallelism between masses of lipase in different kinds of tissues and fat synthesis. Loevenhart indicated that this might be a fact, but a more extensive investiga­tion by H. C. Bradley has made this very dubious.21

Very little is known concerning the reversible action of the hydrolytic protein enzymes. A. E. Taylor digested protamine sulphate with trypsin and found that after adding trypsin to the products of diges­tion a precipitate was formed after long standing; and we may also refer to experi­ments of Robertson with pepsin on the products of caseinogen to which we shall return in the next chapter. It therefore looks at present as if van’t Hoff’s idea of reversible enzyme action might hold in the modifica­tion offered by Armstrong. It remains doubtful, however, whether this reversibility can explain all the synthetic processes in the cell. No objec­tion can be offered at present if any one makes the assump­tion that each cell has specific synthetic enzymes or some other synthetic mechanisms which are still unknown.

The mechanisms for the synthesis of proteins must have one other peculiarity: they must be specific in their action. We shall see in the next chapter that each species seems to possess one or more proteins not found in any other but closely related species. Each organism develops from a tiny microscopic germ and grows by synthetizing the non-specific building stones (amino acids) into the specific proteins of the species. This must be the work of the yet unknown synthetic enzymes or mechanisms. The elucida­tion of their character would seem one of the main problems of biology. Needless to say crystallography is not confronted with problems of such a nature.

The fact that the living cell grows after taking up food has given rise to curious misunderstandings. Traube has shown that drops of a liquid surrounded with a semipermeable membrane may increase in volume when put into a solu­tion of lower osmotic pressure. This has led and is possibly still leading to the statement that the process of growth by a living cell has been imitated artificially. Only one feature has been imitated, the increase in volume; but the essential feature of the process in the living cell, i. e., the forma­tion of the specific constituents of the living cell from non-specific products, has of course not been imitated.

4. The constant synthesis then of specific material from simple compounds of a non-specific character is the chief feature by which living matter differs from non-living matter. With this character is correlated another one; namely, when the mass of a cell reaches a certain limit the cell divides. This is perhaps most obvious in bacteria which on the proper nutritive medium take up food, grow, and divide into two bacteria, each of which takes up food, divides, and grows ad infinitum, as long as the food lasts, provided the harmful products of metabolism are removed. If it be true that specific synthetic ferments exist in each cell it follows that the cell must synthetize these also,22 as otherwise the synthesis of specific proteins would have to come to a standstill.

This problem of synthesis leads to the assump­tion of immortality of the living cell, since there is no a priori reason why this synthesis should ever come to a standstill of its own accord as long as enough food is available and the proper outside physical condi­tions are guaranteed. It is well known that Weismann has claimed immortality for all unicellular organisms and for the sex cells of metazoa, while he claimed the necessity of death for the body cells of the latter. Leo Loeb was led by his investiga­tions on the transplanta­tion of cancer to assume immortality not only for the cancer cell but also for the body cell of the organism. He had found in transplanting a malignant tumor from one individual to another that the tumor grew; that it was not the cells of the host but the transplanted tumor cells of the graft which grew and multiplied, and that this process could be repeated apparently indefinitely so that it was obvious that the transplanted tumor cells outlived the original animal. Such experi­ments have since been carried on so long that we may now say that an individual cancer cell taken from an animal and transplanted from time to time on a new host lives apparently indefinitely. Leo Loeb had found that these tumor cells are simply modified somatic cells. He therefore suggested that the somatic cells might be considered immortal with the same right as we speak of the immortality of the germ cells of such animals.23

This view receives its support first from the fact that certain trees like the Sequoia live several thousand years and may therefore be considered immortal and second, from the method of tissue culture. The method of cultivating tissue cells in a test tube, in the same way as is done for bacteria, was first proposed and carried out by Leo Loeb, in 1897,24 but his test-tube method did not permit the observa­tion of the transplanted cell under the microscope. This was made possible by a modifica­tion of the method by Harrison, who established the fact that the axis cylinder grows out from the ganglionic cell. Harrison and Burrows then perfected the method for the cultiva­tion of the cells of warm-blooded, animals, and with the aid of these methods Carrel succeeded in keeping connective-tissue cells of the heart of an early chick embryo alive more than four years, and these cells are still growing and dividing.25 Only very tiny masses of cells can be kept alive in this way since all the cells in the centre of a piece die on account of lack of oxygen; and every two days a few cells from the margin of the piece have to be transferred to a new culture medium.

This effect of lack of oxygen explains also why the immortality of the somatic cells is not obvious. Death in a human being consists in the stopping of heart beat and respira­tion, which also terminates the action of the brain or at least of consciousness. Immediately after the cessa­tion of heart beat and respira­tion the cells of muscle and of the skin and probably many or most other organs are still alive and might continue to live if transferred to another body with circula­tion and respira­tion. As a consequence of the lack of oxygen supply in the dead body they will, however, die comparatively rapidly. It may be stated that hearts taken out of the body after a number of hours can still beat again when put into the proper solu­tions and upon receiving an adequate oxygen supply.

The idea that the body cells are naturally immortal and die only if exposed to extreme injuries such as prolonged lack of oxygen or too high a temperature helps to make one problem more intelligible. The medical student, who for the first time realizes that life depends upon that one organ, the heart, doing its duty incessantly for the seventy years or so allotted to man, is amazed at the precariousness of our existence. It seems indeed uncanny that so delicate a mechanism should func­tion so regularly for so many years. The mysticism connected with this and other phenomena of adapta­tion would disappear if we could be certain that all cells are really immortal and that the fact which demands an explana­tion is not the continued activity but the cessa­tion of activity in death. Thus we see that the idea of the immortality of the body cell if it can be generalized may be destined to become one of the main supports for a complete physico­chemical analysis of life phenomena since it makes the durability of organisms intelligible.

5. This generalized idea of the immortality of some or possibly most or all somatic cells has a bearing upon the problem of the origin of life on our planet. The experi­ments of Spallanzani, Schwann, Schroeder, Pasteur, Tyndall, and all those who have worked with pure cultures of micro-organisms, have proved that no spontaneous genera­tion of living from non-living matter can be demonstrated; and the statements to the contrary were due to experi­mental errors inasmuch as the new organisms formed were the offspring of others which had entered into the culture medium by mistake.

In the last chapter of that most fascinating book Worlds in the Making,26 Arrhenius discusses the possibility of life being eternal and of living germs of very small dimensions—e. g., the spores of micro-organisms—being carried through space from one planet to another or even from one solar system to another. If it be true that there is no spontaneous genera­tion; if it be true that all cells are potentially immortal, we may indeed seriously raise the ques­tion: May not life after all be eternal? Such ideas were advocated by Richter in a rather phantastic way and more definitely by Helmholtz as well as Kelvin. The latter authors assumed that in the collision of planets or worlds on which there is life, fragments containing living organisms will be torn off and these fragments will move as seed-bearing stones through space. “If at the present instant no life existed upon this earth, one such stone falling upon it might . . . lead to its becoming covered with vegeta­tion.” Arrhenius points out the difficulties which oppose such a view, as, e. g., the fact “that the meteorite in its fall towards the earth becomes incandescent all over its surface and any seeds on it would therefore be deprived of their germinating power.”

Arrhenius suggests another and much more ingenious idea based on the fact that for particles below a certain size the mechanical pressure produced by light waves—the radia­tion pressure—can overcome the attractive force of gravita­tion.

This assumption is undoubtedly correct.

For the assump­tion of eternity of life only the transference of germs from one solar system to another would have to be considered and the ques­tion arises whether or not germs can keep their vitality so many thousands of years. Arrhenius thinks that this is possible on account of the low temperature (which must be below -220° C.) at which no chemical reac­tion and hence no decomposi­tion and deteriora­tion are possible in the spores; and on account of the absence of water vapour.

The ques­tion then arises: Have we any facts to warrant the assump­tion that spores may remain alive for thousands of years under such condi­tions and retain their power of germina­tion? We know that seeds have a very limited vitality, and the statement that grain found in the Egyptian tombs was still able to germinate has long been recognized as a myth. Miss White27 found that in wheat grains, there appeared a well-marked drop in their germinating power after about the fourth year, reaching zero in eleven to seventeen years. In a drier climate they last longer than in a moist climate. It is of importance that the hydrolyzing enzymes in the seeds, such as diastase, erepsin, remained unimpaired even after the germinating power of the seeds had disappeared. The seeds were able to resist for two days the temperature of liquid air, though the subsequent germina­tion was delayed by this treatment. Macfadyen28 exposed non-sporing bacteria, viz., B. typhosus, B. coli communis, Staphylococcus pyogenes aureus, and a Saccharomyces to liquid air.

Paul Becquerel29 found that seeds which possess a very thick integument may live longer than the grain in Miss White’s experi­ments. The thickness of the integument prevents the exchange of gases between air and seed. Thus seeds of leguminoses (Cassia bicapsularis, Cytisus biflorus, Leucæna leucocephala, and Trifolium arvense) had retained their power of germina­tion for eighty-seven years. Becquerel has shown that the dryness of the membrane is very essential for such a dura­tion of life, since when dry it is impermeable for gases and the slow chemical reac­tions inside the grain become impossible.

In the cosmic space there is no water vapour, no atmosphere, and a low temperature, and there is hence no reason why spores should lose appreciably more of their germinating power in ten thousand years than in six months. We must therefore admit the possibility that spores may move for an almost infinite length of time through cosmic space and yet be ready for germina­tion when they fall upon a planet in which all the condi­tions for germina­tion and development exist, e. g., water, proper temperature, and the right nutritive substances dissolved in the water (inclusive of free oxygen).

While thus everything is favourable to Arrhenius’s hypothesis, Becquerel raises the objec­tion that the spores going through space would yet be destroyed by ultraviolet light. This danger would probably exist only as long as the germ is not too far from a sun. The difficulty is a real one since the ultraviolet rays have a destructive effect even in the absence of oxygen. It is possible, however, that there are spores which can resist this effect of ultraviolet light. Arrhenius’s theory can not of course be disproved and we must agree with him that it is consistent not only with the theories of cosmogony but also with the seeming potential immortality of certain or of all cells.

The alternative to Arrhenius’s theory is that living matter did originate and still originates from non-living matter. If this idea is correct it should one day be possible to discover synthetic enzymes which are capable of forming molecules of their own kind from a simple nutritive solu­tion. With such synthetic enzymes as a starting point the task might be undertaken of creating cells capable of growth and cell division, at least in the apparently simple form in which these phenomena occur in bacteria; viz., that after the mass has reached a certain (still microscopic) size it divides into two cells and so on. If Arrhenius is right that living matter has had no more beginning than matter in general, this hope of making living matter artificially appears at present as futile as the hope of making molecules out of electrons.

The problem of making living matter artificially has been compared to that of constructing a perpetuum mobile; this comparison is, however, not correct. The idea of a perpetuum mobile contradicts the first law of thermodynamics, while the making of living matter may be impossible though contradicting no natural law.

Pasteur’s proof that spontaneous genera­tion does not occur in the solu­tions used by him does not prove that a synthesis of living from dead matter is impossible under any condi­tions. It is at least not inconceivable that in an earlier period of the earth’s history radio-activity, electrical discharges, and possibly also the action of volcanoes might have furnished the combina­tion of circumstances under which living matter might have been formed. The staggering difficulties in imagining such a possibility are not merely on the chemical side—e. g., the produc­tion of proteins from CO₂, and N—but also on the physical side if the necessity of a definite cell structure is considered. We shall see in the sixth chapter that without a structure in the egg to begin with, no forma­tion of a complicated organism is imaginable; and while a bacterium may have a simple structure, such a structure as it possesses is as necessary for its existence as are its enzymes.

Attempts have repeatedly been made to imitate the structures in the cell and of living organisms by colloidal precipitates. It is needless to point out that such precipitates are of importance only for the study of the origin of structures in the living, but that they are not otherwise an imita­tion of the living since they are lacking the characteristic synthetic chemical processes.

CHAPTER III

THE CHEMICAL BASIS OF GENUS AND SPECIES

1. It is a truism that from an egg of a species an organism of this species only and of no other will arise. It is also a truism that the so-called protoplasm of an egg does not differ much from that of eggs of other species when looked at through a microscope. The ques­tion arises: What determines the species of the future organism? Is it a structure or a specific chemical or groups of chemicals? In a later chapter we shall show that the egg has a simple though definite structure, but in this chapter we shall see that the egg must contain specific substances and that these substances which determine the “species” and specificity in general are in all probability proteins. Since solu­tions of different proteins look alike under a microscope we need not wonder that it is impossible to discriminate microscopically between the protoplasm of different eggs.

The idea of definiteness and constancy of species, a matter of daily observa­tion in the case of man and higher animals in general, was not so readily accepted in the case of the micro-organisms, which on account of their minuteness and simplicity of structure are not so easy to differentiate. There existed for a long time serious doubt whether or not the simplest organisms, the bacteria, possessed a definite “specificity” like the higher organisms, or whether they were not endowed, as Warming put it, with an “unlimited plasticity,” which forbade classifying them according to their form into definite species as Cohn had done. An interesting episode in this discussion, which was settled about twenty-five years ago arose concerning the sulphur bacteria, which often develop in large masses on parts of decaying plants or animals along the shore. Sir E. Ray Lankester found collec­tions of red bacteria covering putrefying animal matter in a vessel and forming a continuous membrane along its wall. These red bacteria were of very different shape, size, and grouping, but they seemed to be connected by transi­tion forms. They had a common character, however, namely, their peach-coloured appearance. This common character, together with their associa­tion in the same habitat, led Lankester to the then justifiable belief that they all belonged to one species which was protean in character and that the different forms were only to be considered as phases of growth of this one species. The presence of the same red pigment “Bacterio-purpurin” seemed justly to indicate the existence of common chemical processes. Cohn, on the contrary, considered the different forms among these red bacteria (they are today called sulphur bacteria since they oxidize the hydrogen sulphide produced by bacteria of putrefac­tion to sulphur and sulphates) as definite and distinct species, in spite of their common colour and their associa­tion. Later observa­tions showed that Cohn was right. Winogradsky30 succeeded in proving by pure culture experi­ments that each of these different forms of sulphur bacteria was specific and did not give rise to any of the other forms of the same colour found in the same condi­tions.

The method of pure line breeding inaugurated by Johannsen31 has shown that the degree of definiteness goes so far that apparently identical forms with only slight differences in size may breed true to this size; but for reasons which will become clear later on we may doubt whether they are to be considered as definite species.

The fact of specificity is supported by the fact of constancy of forms. de Vries has pointed out that regardless of the possible origin of new species by muta­tion the old species may persevere. Walcott has found fossils of annelids, snails, crustaceans, and algæ in a precambrian forma­tion in British Columbia whose age (estimated on the rate of forma­tion of radium from uranium) may be about two hundred million years and estimated on the basis of sedimenta­tion sixty million years. And yet these invertebrates are so closely related to the forms existing today that the systematists have no difficulty in finding the genus among the modern forms into which each of these organisms belongs. W. M. Wheeler, in his investiga­tions of the ants enclosed in amber, was able to identify some of them with forms living today, though the ants observed in the amber must have been two million years old. The constancy of species, i. e., the permanence of specificity may therefore be considered as established as far back as two or possibly two hundred millions of years. The definiteness and constancy of each species must be determined by something equally definite and constant in the egg, since in the latter the species is already fixed irrevocably.

We shall show first that species if sufficiently separated are generally incompatible with each other and that any attempt at fusing or mixing them by grafting or cross-fertilizing is futile. In the second part of the chapter we shall take up the facts which seem destined to give a direct answer to the ques­tion as to the cause of specificity. It is needless to say that this latter ques­tion is of paramount importance for the problem of evolu­tion, as well as for that of the constitu­tion of living matter.

I. The Incompatibility of Species not closely Related

2. It is practically impossible to transplant organs or tissues from one species of higher animals to another, unless the two species are very closely related; and even then the transplanta­tion is uncertain and the graft may either fall off again or be destroyed. This specificity of tissues goes so far that surgeons prefer, when a transplanta­tion of skin in the human is intended, to use skin of the patient or of close blood rela­tions. The reason why the tissues of a foreign species in warm-blooded animals cannot grow well on a given host has been explained by the remarkable experi­ments of James B. Murphy of the Rockefeller Institute.32 Murphy discovered that it is possible to transplant successfully any kind of foreign tissue upon the early embryo of the chick. Even human tissue transplanted upon the chick embryo will grow rapidly. This shows that at this early stage the chick embryo does not yet react against foreign tissue. This lack of reac­tion lasts until about the twenty-first day in the life of the embryo; then the growth of the graft not only ceases but the graft itself falls off or is destroyed. Murphy noticed that this critical period coincides with the development of the spleen and of lymphatic tissue in the chick and that a certain type of migrating cells, the so-called lymphocytes, which develop in the lymphatic tissue, gather at the edge of the graft in great numbers, and he suggested that these lymphocytes (by a secre­tion of some substance?) rid the host of the graft. He applied two tests both of which confirmed this idea. First he showed that when small fragments of the spleen of an adult chicken are transplanted into the embryo the latter loses its tolerance for foreign grafts. The second proof is still more interesting. It was known that by treatment with Roentgen rays the lymphocytes in an animal could be destroyed. It was to be expected that an animal so treated would have lost its specific resistance to foreign tissues. Murphy found that this was actually the case. On fully grown rats in which the lymphocytes had been destroyed by X-rays (as ascertained by blood counts) tissues of foreign species grew perfectly well. These experi­ments have assumed a great practical importance since they can also be applied to the immuniza­tion of an animal against transplanted cancer of its own species. Murphy found that by increasing the number of lymphocytes in an animal (which can be accomplished by a mild treatment with X-rays) the immunity against foreign grafts as well as against cancer from the same species can be increased. It is quite possible that the apparent immunity to a transplanta­tion of cancer produced by Jensen, Leo Loeb, and Ehrlich and Apolant through the previous transplanta­tion of tissue in such an animal was due to the fact that this previous tissue transplanta­tion led to an increase in the number of lymphocytes in the animal. The medical side, however, lies outside of our discussion, and we must satisfy ourselves with only a passing notice. The facts show that each warm-blooded animal seems to possess a specificity whereby its lymphocytes destroy transplanted tissue taken from a foreign species.

A lesser though still marked degree of incompatibility exists also in lower animals for grafts from a different species.33 The graft may apparently take hold, but only for a few days, if the species are not closely related. Joest apparently succeeded in making a permanent union between the anterior and posterior ends of two species of earthworms, Lumbricus rubellus and Allolobophora terrestris. Born and later Harrison healed pieces of tadpoles of different species together. An individual made up of two species Rana virescens and Rana palustris lived a considerable time and went through metamorphosis. Each half regained the characteristic features of the species to which it belonged. It seems, however, that if species of tadpoles of two more distant species are grafted upon each other no lasting graft can be obtained, e. g., Rana esculenta and Bombinator igneus. These experi­ments were made at a time when the nature and bearing of the problem of specificity was not yet fully recognized. The rôle of lymphocytes in these cases has never been investigated. The grafted piece always retained the characteristics of the species from which it was taken.

Plants possess no leucocytes and we therefore see that they tolerate a graft of foreign tissues better than is the case in animals. As a matter of fact hetero­c grafting is a common practice in horticulture, although even here it is known that indiscriminate hetero­plastic grafting is not feasible and that therefore the specificity is not without influence. The host is supposed to furnish only nutritive sap to the graft and in this respect does not behave very differently from an artificial nutritive solu­tion for the raising of a plant. The law of specificity, however, remains true also for the grafted tissues: neither in animals nor in plants does the graft lose its specificity, and it never assumes the specific characters of the host, or vice versa. The apparent excep­tions which Winkler believed he had found in the case of grafts of nightshade on tomatoes turned out to be a further proof of the law of specificity. Winkler, after the graft had taken, cut through the place of grafting, after which opera­tion a callus forma­tion occurred on the wound. In most cases either a pure nightshade or a pure tomato grew out from this callus. In some cases he obtained shoots from the place where graft and host had united, which on one side were tomato, on the other side nightshade. What really happened was that the shoots had a growing point whose cells on the one side consisted of cells of nightshade, on the other side of tomato.34 We know of no case in which the cell of a graft has lost its specificity and undergone a trans­forma­tion into the cell of the host.

3. Another manifesta­tion of the incompatibility of distant species is found in the domain of fertiliza­tion. The eggs of the majority of animals cannot develop unless a spermato­zoön enters. The entrance of a spermato­zoön into an egg seems also to fall under the law of specificity, inasmuch as in general only the sperm of the same or a closely related species is able to enter the egg. The writer35 has found, however, that it is possible to overcome the limita­tion of specificity in certain cases by physico­chemical means, and by the knowledge of these means we may perhaps one day be able to more closely define the mechanism of specificity in this case. He found that the eggs of a certain Californian sea urchin, which cannot be fertilized by the sperm of starfish in normal sea water, will lose their specificity towards this type of foreign sperm if the sea water is rendered a little more alkaline, or if a little more Ca is added to the sea water, or if both these varia­tions are effected. Godlewski has confirmed the efficiency of this method for the fertiliza­tion of sea-urchin eggs with the sperm of crinoids.

If such hetero­geneous hybridiza­tions are carried out, two striking results are obtained. The one is that the resulting larva has only maternal characteristics (Figs. 1 and 2), as if the sperm had contributed no hereditary material to the developing embryo. This result could not have been predicted, for if we fertilize the egg of the same Californian sea urchin, Strongylo­centrotus purpuratus, with the sperm of a very closely related sea urchin, S. franciscanus, the hereditary effect of the spermato­zoön is seen very distinctly in the primitive skeleton formed by the larva.36 (Fig. 3.) In the case of the hetero­geneous hybridiza­tion the spermato­zoön acts practically only as an activating agency upon the egg and not as a transmitter of paternal qualities.

The second striking fact is that while the sea-urchin eggs fertilized with starfish sperm develop at first perfectly normally they begin to die in large numbers on the second and third day of their development, and only a very small number live long enough to form a skeleton; and these are usually sickly and form the skeleton considerably later than the pure breed. It is not quite certain whether the sickliness of these hetero­geneous hybrids begins or assumes a severe character with the development of a certain type of wandering cells, the mesenchyme cells; it would perhaps be worth while to investigate this possibility. The writer was under the impression that this sickliness might have been brought about by a poison gradually formed in the hetero­geneous larvæ.

He investigated the effects of hetero­geneous hybridiza­tion also in fishes, which are a much more favourable object. The egg of the marine fish Fundulus hetero­clitus can be fertilized with the sperm of almost any other teleost fish, as Moenkhaus37 first observed. This author did not succeed in keeping the hybrids alive more than a day, but the writer has kept many hetero­geneous hybrids alive for a month or longer,38 and found the same two striking facts which he had already observed in the hetero­geneous cross between sea urchin and starfish: first, practically no transmission of paternal characters, and second, a sickly condi­tion of the embryo which begins early and which increases with further development. The hetero­geneous fish hybrids between, e. g., Fundulus hetero­clitus ♀ and Menidia ♂ have usually no circula­tion of blood, although the heart is formed and beats and blood-vessels and blood cells are formed; the eyes are often incomplete or abnormal though they may be normal at first; the growth of the embryo is mostly retarded. In excep­tional cases circula­tion may be established and in these a normal embryo may result, but such an embryo is chiefly maternal.

This incompatibility of two gametes from different species does not show itself in the case of hetero­geneous hybridiza­tion only, but also though less often in the case of crossing between two more closely related forms. The cross between the two related forms S. purpuratus ♀ and S. franciscanus ♂ is very sturdy and shows no abnormal mortality as far as the writer’s observa­tions go. If, however, the reciprocal crossing is carried out, namely that of S. franciscanus ♀ and S. purpuratus ♂, the development is at first normal, but beginning with the time of mesenchyme forma­tion the majority of larvæ become sickly and die; and again the ques­tion may be raised whether or not the beginning of sickliness coincides with the development of mesenchyme cells. If we assume that the sickliness and death are due to the forma­tion of a poison, we must assume that the poison is formed by the protoplasm of the egg, since otherwise we could not understand why the reciprocal cross should be healthy.

All of these data agree in this one point, that the fusion by grafting or fertiliza­tion of two distant species is impossible, although the mechanism of the incompatibility is not yet understood. It is quite possible that this mechanism is not the same in all the cases mentioned here, and that it may be different when two different species are mixed and when incompatibility exists between varieties, as is the case in the graft on mammals.

II. The Chemical Basis of Genus and Species and of Species Specificity

4. Fifty or sixty years ago surgeons did not hesitate to transfuse the blood of animals into human beings. The practice was a failure, and Landois39 showed by experi­ment that if blood of a foreign species was introduced into an animal the blood corpuscles of the transfused blood were rapidly dissolved and the animal into which the transfusion was made was rendered ill and often died. The result was different when the animals whose blood was used for the purpose of transfusion belonged to the same species or a species closely related to the animal into which the blood was transfused. Thus when blood was exchanged between horse and donkey or between wolf and dog or between hare and rabbit no hemoglobin appeared in the urine and the animal into which the blood was transfused remained well.40 This was the beginning of the investiga­tions in the field of serum specificity which were destined to play such a prominent rôle in the development of medicine. Friedenthal was able to show later that if to 10 c.c. of serum of a mammal three drops of defibrinated blood of a foreign species are added and the whole is exposed in a test tube to a temperature of 38°C. for fifteen minutes the blood cells contained in the added blood are all cytolyzed; that this, however, does not occur so rapidly when the blood of a related species is used. He could thus show that human blood serum dissolves the erythrocytes of the eel, the frog, pigeon, hen, horse, cat, and even that of the lower monkeys but not that of the anthropoid apes. The blood of the chimpanzee and of the human are no longer incompatible, and this discovery was justly considered by Friedenthal as a confirma­tion of the idea of the evolu­tionists that the anthropoid apes and the human are blood rela­tions.41

This line of investiga­tion had in the meanwhile entered upon a new stage when Kraus, Tchistowitch, and Bordet discovered and developed the precipitin reac­tion, which consists in the fact that if a foreign serum (or a foreign protein) is introduced into an animal the blood serum of the latter acquired after some time the power of causing a precipitate when mixed with the antigen, i. e., with the foreign substance originally introduced into the animal for the purpose of causing the produc­tion of antibodies in the latter; while, of course, no such precipita­tion occurs if the serum of a non-treated rabbit is mixed with the serum of the blood of the foreign species.

In 1897 Kraus discovered that if the filtrates from cultures of bacteria (e. g., typhoid bacillus) are mixed with the serum of an animal immunized with the same serum (e. g., typhoid serum) it causes a precipitate; and that this precipitin reac­tion is specific. This fact was confirmed and has been extended by the work of many authors.

Tchistowitch in 1899 observed that the serum of rabbits which had received injec­tions of horse or eel serum caused a precipitate when mixed with the serum of these latter animals.

Bordet found in 1899 that if milk is injected into a rabbit the serum of such a rabbit acquires the power of precipitating casein, and Fish found that this reac­tion is specific inasmuch as the lactoserum from cow’s milk can precipitate only the casein of cow’s milk but not that of human or goat milk. Wassermann and Schütze reached the same result independently of each other.

Myers and later Uhlenhuth showed that if white of egg from a hen’s egg is injected into a rabbit, precipitins for white of egg are found in the serum of the latter, and Uhlenhuth42 found, by trying the white of egg of different species of birds, that the precipitin reac­tion called forth by the blood of the immunized animal is specific, inasmuch as the proteins from a hen’s egg will call forth the forma­tion of precipitins in the blood of the rabbit which will precipitate only the white of egg of the hen or of closely related birds.

To Nuttall43 belongs the credit of having worked out a quantitative method for measuring the amount of precipitate formed, and in this way he made it possible to draw more valid conclusions concerning the degree of specificity of the precipitin reac­tion. He found by this method that when the immune serum is mixed with the serum or the protein solu­tion used for the immuniza­tion a maximum precipitate is formed, but if it is mixed with the serum of related forms a quantitatively smaller precipitate is produced. In this way the degree of blood rela­tionship could be ascertained. He thus was able to show that when the blood of one species, e. g., the human, was injected into the blood of a rabbit, after some time the serum of the rabbit was able to cause a precipitate not only with the serum of man, or chimpanzee, but also of some lower monkeys; with this difference, however, that the precipitate was much heavier when the immune serum was added to the serum of man. The method thus shows the existence of not an absolute but of a strong quantitative specificity of blood serum. This statement may be illustrated by the following table from Nuttall. The antiserum used for the precipitin reac­tion was obtained by treating a rabbit with human blood serum. The forty-five bloods tested had been preserved for various lengths of time in the refrigerator with the addi­tion of a small amount of chloroform.

TABLE II

Quantitative Tests with Anti-Primate Sera

Tests with Antihuman Serum