As shown in the two preceding chapters unicellular organisms and plants afford evidence of numerous phenomena of immunity. Alongside natural immunity we find in them undoubted evidence of an adaptation to the presence of morbific agents, evidence which warrants us in inferring that cases of acquired immunity are frequent. This being the case it is quite natural that the animal kingdom should be no exception to the general rule. Indeed, immunity against pathogenic agents is widely distributed in animals, and we continually see manifestations of natural immunity not only against parasites and their toxins, but against poisons in general. Just as frequently we find cases of acquired immunity against these morbific agents.

As yet we know but little concerning the phenomena of immunity in the lower animals belonging to the great group of the Invertebrata. But it may be affirmed with certainty that these also are often endowed with a natural immunity against micro-organisms and bacterial toxins. As an example I may cite the case of the large white larvae of the Rhinoceros beetle (Oryctes nasicornis) frequently met with in tanner’s bark. Very susceptible to the cholera vibrio—¹⁄₈₀₀₀ of a culture^[53] of this organism being sufficient to set up a fatal septicaemia—these larvae exhibit a very remarkable natural immunity against the bacilli of anthrax and diphtheria. A large dose of bacteria of the second anthrax vaccine, fatal to rabbits, guinea-pigs and mice, is borne without any inconvenience by the larvae of the Rhinoceros beetle. They are equally refractory to large doses of the diphtheria bacillus. And yet, there are not wanting species of insects which are susceptible to these same micro-organisms. Thus, according to A. Kovalevsky^[54], crickets contract anthrax very readily even at moderate temperatures (22°–23° C.). On the other hand they are, according to the same author, refractory to the bacillus of avian tuberculosis. Many of the Invertebrata, studied from this point of view, present analogous facts, with which, however, we need not at present occupy ourselves.

In the Vertebrata in general and in Man in particular, natural immunity against many infective diseases and soluble poisons is so widespread that we are at no loss to find examples for citation. We have a whole series of human infections whose study is rendered particularly difficult simply because of the natural immunity of all other species of animals from these infections. Such are syphilis, scarlatina, leprosy, exanthematous typhus, etc. On the other hand, a large number of diseases, very infective for domestic animals, are quite innocuous to man. In this group we have cattle plague, strangles, contagious pleuro-pneumonia, fowl cholera, pneumo-enteritis of pigs, and a number of other diseases.

As in a very large majority of instances pathogenic organisms act through the agency of their toxic products, one might believe—and this has been assumed repeatedly—that natural immunity against infective diseases is dependent on the insusceptibility of the refractory organism to the specific poisons.

Such a supposition cannot survive criticism. We have undoubted instances of a species of animal being resistant both to a micro-organism and to its toxin. Such instances, however, are rare and usually an organism that is refractory or only slightly susceptible to the micro-organism itself is very susceptible to its toxic products. Even those micro-organisms which come almost constantly in contact with the human organism without becoming pathogenic, may produce toxins capable of gravely affecting health. Let us take as an example the bacillus of blue pus. This organism is most widely diffused in human surroundings. According to Schimmelbusch^[55] it is met with on the skin of the arm-pits and of the inguinal region of one-half of mankind. From the skin it very often passes into the dressings of wounds which then assume the characteristic and so long recognised blue colour. The same bacillus is also found in the intestines of both sick and healthy persons. Jakowski^[56] has met with it in the faeces coming from intestinal fistulae in two women who had undergone operations. Now, in spite of these specially favourable conditions for the production of infection, the Bacillus pyocyaneus has remained harmless. It is only in children, and even then rarely, that it can be convicted of exciting disease. Man, then, usually enjoys a true natural immunity against the Bacillus pyocyaneus. And yet it is not to his insusceptibility to the pyocyanic toxin that he is indebted for this immunity. Schaffer^[57], having injected himself in the shoulder with half a c.c. of a sterilised culture of B. pyocyaneus, developed fever and an erysipelatous swelling. Bouchard and Charrin^[58] injected pyocyanic toxin into patients who reacted with more or less fever and by other toxic symptoms.

Another extremely common saprophyte, the Micrococcus prodigiosus, is incapable of setting up an infective disease, but this does not prevent its products from exercising a toxic action, often very grave, in man. The frog, which is refractory to the cholera vibrio, undergoes a fatal intoxication when cholera toxin is injected. One of the most striking examples is furnished in the case of the human tubercle bacillus and tuberculin. Man is much more resistant than is the guinea-pig to the pathogenic action of this organism, yet he is incomparably more susceptible to its toxin (tuberculin). According to the researches of Behring and Kitashima^[59], the sheep, of all species of mammals, is most susceptible to the tubercular poison; the Bovidae and the guinea-pig occupy an inferior rank in the scale of susceptibility. On the other hand, the guinea-pig is very susceptible to the tubercle bacillus; the Bovidae are less so and the sheep is still more resistant to tuberculosis. It is unnecessary to multiply instances. Immunity against microbial infection and against intoxication are two distinct properties, so that it is impossible to reduce the former to an insusceptibility to toxins. We must therefore consider these two kinds of immunity separately and we will first consider the resistance of the animal organism against living infective micro-organisms.

Refractory human beings and animals may be inoculated with a large number of micro-organisms without being affected. Thus Opitz^[60] injected 10,000,000 organisms into the blood of a dog. Twenty minutes later he could find no more than 9000. It is then quite natural to ask, What becomes of these micro-organisms after they have made their way into the interior of the refractory organism? It has been suggested that the animal gets rid of the pathogenic germs much as it does of all kinds of soluble poisons. Certain of these poisons, such as iodine and alcohol, are in great part eliminated by the kidneys; others, such as iron, by the alimentary canal. Why, it is asked, should not micro-organisms also be eliminated by the same channels? Flügge has adopted this view and has expounded it in his work on ferments and micro-organisms^[61]. Moreover he suggested to Wyssokowitch^[62] that he should carry out a large series of experiments with the object of verifying this theory. But numerous very careful researches have given a result quite at variance with the forecast made by Flügge. Micro-organisms of various species, injected into the blood vessels of rabbits and dogs, were, in those cases where these animals are refractory, never eliminated, either by the kidneys or by any other of the excretory channels which were studied. When bacteria pass into the secretions, lesions of the tissues, more or less grave, are invariably present.

This result has been repeatedly confirmed and has been accepted as a general experience. The elimination of micro-organisms by the urine indicates not merely the absence of immunity, but implies, also, a susceptibility of the organism. In many septicaemias, such as those produced by the anthrax bacillus, the streptococcus and other bacteria, or in less generalised diseases, such as typhoid fever, bacteria are found in the urine, often in large numbers. In these cases it is a question of anything but a refractory condition even of the slightest degree.

In recent years, however, several works have been published the aim of which was to demonstrate the inaccuracy of this apparently well-established thesis. Biedl and Kraus^[63] in Vienna took the initiative and announced in a detailed work that micro-organisms can readily pass intact into the kidney and that this organ in virtue of its physiological function eliminates them. The organisms were said to leave the blood capillaries by the normal process of diapedesis and were then eliminated with the urine. The liver in a physiological condition, according to the researches of these authors, is equally capable of allowing of the passage of micro-organisms; indeed it aids in discharging them from the system. On the other hand, the pancreas and the salivary glands were incapable of fulfilling this function. Von Klecki^[64] obtained similar results. He also holds that the kidney is the principal organ of elimination for micro-organisms which have penetrated into a refractory organism.

With these contradictions before him, Opitz^[65] set himself to study this question in Flügge’s laboratory at Breslau. Having critically reviewed the technical methods of his predecessors and carried out a series of new experiments, he declared categorically “that a physiological excretion, by the kidneys, of the micro-organisms which circulate in the blood, does not exist.” For Opitz “the frequent appearance of micro-organisms in the urine of animals into whose blood, a short time previously, living bacteria have been injected, is due to mechanical and chemical lesions of the vessel wall and of the renal epithelia.”

This question might be looked upon as definitely settled in favour of the first results obtained by Wyssokowitch were it not that other voices had been raised in favour of a physiological excretion of the micro-organisms by the renal channels. Pawlowsky^[66] has recently published a long work on this subject in which he attempts to demonstrate that certain micro-organisms, even when introduced into the subcutaneous tissue of animals, pass very rapidly (at the end of a quarter of an hour) into the uropoietic organs and are eliminated with the urine.

It was necessary to put an end to these controversies and Métin^[67] undertook a series of researches at the Pasteur Institute with the object of clearing up this question. He guarded himself against the objections justly made against his predecessors and conducted his experiments under unexceptionable conditions. He injected several species of micro-organisms into the veins of rabbits and into the subcutaneous tissue of guinea-pigs. At various intervals he performed laparotomy on these animals, pulled out the bladder and drew off the urine in such a fashion that no trace of blood could get into it. The results were most conclusive. Never, when the experiment was conducted under the rigorous conditions just mentioned, did the micro-organisms traverse the kidneys of resistant animals nor were they ever met with in their urine.

Métin’s researches on the passage of micro-organisms through the liver in refractory animals gave the same results. In no case was he able to find in the bile any of the organisms that had been injected into the blood or under the skin. At the end of his memoir Métin sums up his results as follows: “(1) The kidneys and the liver are impermeable to bacteria introduced into the organism, subcutaneously or intravenously; (2) when the culture tubes contain colonies of the injected micro-organism, it is because there has been a certain amount of blood in the fluid inoculated, this being an indication of a vascular or epithelial lesion, either mechanical or chemical.” We were present at M. Métin’s experiments and can bear witness to their exactitude.

There can no longer be any doubt then on this point. The elimination of the micro-organisms from the refractory animal takes place, as indicated in Wyssokowitch’s first investigation, neither by the kidneys nor by the liver. Some observers have asserted that this elimination may take place by the sudoriparous glands. Thus, Brunner^[68] made experiments with young pigs and cats into which he had previously injected micro-organisms, for the most part pathogenic. Then producing a transpiration by means of pilocarpin, he “cultivated” the sweat and noted the development of the same bacteria as he had introduced into the blood. In a single experiment with a saprophyte (Coccobacillus prodigiosus) he obtained a positive result, from which he concludes that the refractory animal gets rid of bacteria which circulate in its blood by way of the sudoriparous glands. It is scarcely allowable to draw any conclusion from this experiment from the fact that the snout of the pig, the seat of the transpiration, is very liable to small vascular lesions which might furnish the bacteria that developed on Brunner’s plates. Nevertheless, even in the case of pathogenic organisms, which swarm in the blood, the sweat is usually free from them. This has been shown by Krikliwy^[69] in the case of cats inoculated with anthrax whose sweat, in spite of the passage of numerous bacteria into the circulation, contained none.

Micro-organisms, then, after their entrance into the refractory animal, are not eliminated by any of the excretory channels which serve for the elimination of many of the soluble poisons. It was necessary therefore to seek some other process capable of affording an explanation of the disappearance of the micro-organisms which so often and by such varied means make their way into the interior of a resistant organism. For it is a well-established fact that in these cases the micro-organisms do disappear completely. This has been observed so often that it is unnecessary to offer any demonstration of the fact. Perhaps in the refractory organism the micro-organisms undergo the fate of the foreign bodies which penetrate, or which are introduced, into the circulation. It has long been known, thanks especially to the work of Hoffmann and Recklinghausen^[70], and of Ponfick^[71], that particles of carmine or vermilion when injected into the blood are deposited in several organs. They are found in the spleen, the lymphatic glands and the bone-marrow. A certain number of these foreign particles may even be fixed in the liver and kidneys, but, instead of passing into the bile and the urine, they remain lodged in the interstitial tissue of the organs. The observers just cited noted that the coloured granules do not remain long in either the blood or the lymph but will be found in the interior of the cellular elements. These granules persist for weeks without any appreciable modification, differing in this from the micro-organisms which, as a rule, after several days or even after a few hours, disappear from the refractory organism. This disappearance might be more justly compared to the resorption of corpuscular elements which results in a more or less complete atrophy. The facts concerning the resorption of pus, of extravasated blood, of the mucosa of the uterus in pregnancy, etc., have long been known, and it is among these that one should seek analogies with the disappearance of the micro-organisms. When bacteria of various species are injected into refractory or not very susceptible animals, we always observe a local reaction in the form of inflammation, accompanied by the appearance of white corpuscles. Gradually the organisms disappear from the point at which they are introduced; the exudation becomes sterile and ultimately is completely absorbed. Numerous researches, which will be set forth in the succeeding chapters, have, indeed, demonstrated the remarkable analogy that exists between the disappearance of the micro-organisms from the refractory animal and the resorption of corpuscular elements or of animal cells.

The analysis of the phenomena of this resorption will help us considerably in our study of immunity against micro-organisms. When in any part of the animal organism a collection of pus, an effusion of blood, or any other organic lesion is produced, these lesions are usually repaired after the lapse of a longer or shorter interval. In those cases where the cells retain their integrity, they are taken into the lymphatic vessels and then pass into the circulating blood. In the course of his researches on the transfusion of blood, Hayem^[72] observed “that blood injected into the peritoneum is absorbed unaltered and passes with its anatomical elements into the general circulation.” He was able to demonstrate “that the lymphatic channels play an important part in this absorption.” Lesage of Alfort^[73] confirmed this result. He found that in the dog “one hour after an abundant haemorrhage into the peritoneum, induced experimentally, the red corpuscles commenced to pass freely, without alteration and in very large numbers, into the thoracic duct.” I have observed a similar resorption of the red blood corpuscles of the guinea-pig when injected into the peritoneal cavity of other individuals of the same species. The white corpuscles can also be taken up by the lymphatic vessels without being modified in any way. At the end of an inflammatory reaction of feeble intensity, set up in cold-blooded animals, especially in the tadpole, the direct passage of leucocytes from the exudation into the lymphatic system may be observed.

The examples I have just cited are, however, quite exceptional. In the great majority of cases the cellular elements that are undergoing resorption are seized by the amoeboid cells and are taken into their substance. Even in the resorption of the red corpuscles, lying free in the peritoneal cavity of the same species of animal, a certain number of the globules do not pass directly into the circulation but are first ingested by the amoeboid elements. This fact is insisted upon by Lesage. In inflammatory exudations the leucocytes also become the prey of their fellows. The ingested white corpuscles may be recognised for some time lying in the interior of other leucocytes; they are soon broken up, however, and finally disappear completely. When, instead of isolated cells such as leucocytes, we introduce fragments of tissues or of organs into any part of the organism, the same mode of resorption may always be observed. The introduced fragments are first surrounded and infiltrated by amoeboid cells and are then taken up into their interior.

The mode of absorption just described is very general. It applies to all kinds of cells and is observed in the absolutely normal organism, as well as in a large number of pathological conditions. For more than fifty years, the existence of cells which contain red blood corpuscles (“blutkörperchenhaltige Zellen” of German writers) has been recognised; they were met with in the spleen, the lymphatic glands and in many pathological products. For long we could not explain how the red corpuscles come to be inside other cells. Virchow^[74] thought that they got there as the result of a mechanical pressure. Later histologists succeeded in determining the true nature of cells containing red blood corpuscles and in recognising that the leucocytes had really ingested the corpuscles. There has been much discussion, also, on the presence of leucocytes in the interior of large cells in exudations. It was thought that these were mother-cells which contained a new generation of small cells. Writers even described a fusion between the large cell and those found inside it; but Bizzozero^[75] first recognised that the former was an amoeboid cell which had ingested pus corpuscles. Since this observation was made numerous cases have been described in which different cell elements have been found in the large cells. There could no longer be any hesitation in interpreting these cases as instances of ingestion by leucocytes or similar cells.

The changes that the ingested elements undergo within amoeboid cells may be compared with those that take place in intracellular digestion. If the modifications of the particles ingested by the Amoebae be studied side by side with those which take place in ingested cells in the process of resorption, a striking analogy may be observed. To establish this satisfactorily it is essential to begin with a study of intracellular digestion properly so called, especially as in this phenomenon we have the fundamental basis of the whole of the theory developed in this work.

In our first two chapters we have already cited examples of this intracellular digestion in the Protozoa (Amoebae, Infusoria, etc.) and in the plasmodium stage of the Myxomycetes. In all these cases it goes on in the organism, in a distinctly acid medium, by the aid of ferments which could be demonstrated in the Amoebae and Myxomycetes, and which are analogous sometimes with trypsin, sometimes with pepsin.

In the lower Invertebrata we find the principal source of our knowledge of intracellular digestion in the digestive organs. This form of digestion is met with in Sponges, in the whole of the Coelenterates (Medusae, Siphonophora, Ctenophora, etc.), in the great majority of the Turbellaria (Planarians, Rhabdocoela), and in certain of the Mollusca (the lower Gasteropods). In the Invertebrata higher in the animal scale, intracellular digestion in the digestive organs becomes more and more rare, and sometimes it manifests itself only in the larval condition (Phoronis); ultimately it gives place permanently to digestion by juices secreted into the gastro-intestinal canal.

In his sketch of the comparative physiology of digestion, Krukenberg^[76] sought to establish two types: protoplasmic or cellular digestion and secretory digestion. The former is effected, according to this observer, by a vital action independently of any production of soluble ferments. Secretory digestion alone, characteristic of the Vertebrates and of almost all the higher Invertebrates, is effected by means of these ferments (diastases or enzymes). Many observers, adopting this view, maintain that intracellular digestion presents a purely vital phenomenon essentially different from that of chemical digestion due to juices containing soluble ferments secreted in the gastro-intestinal canal. That this theory is absolutely erroneous the succeeding pages of this work will furnish ample proof.

The Protozoa, from their small size, are unsuitable for researches on the essential phenomena of intracellular digestion. Amongst animals higher in the scale the Planarians lend themselves most readily to the observation of this process. These flat worms are very common in both fresh and sea water and are easily fed in captivity. They are very voracious animals and, among other things, devour the blood of man or animals with avidity. One has merely to allow them to fast for a few days, and then to give them a drop of blood in order to see their digestive canal fill itself with this fluid (fig. 6). The white Planarian, Dendrocoelum lacteum, is well adapted for these researches. In a worm that has sucked blood from a Vertebrate, owing to its great transparency, the whole length of its intestine with its numerous ramifications may be seen. For some time this organ remains of a bright red colour, but gradually the tinge becomes brownish or faintly violet. These changes of colour recall those observed in effusions of blood in or under the human skin resulting from contusions. A microscopical examination of Planarians that have been fed with blood shows that the coloration of their digestive canal is due to red blood corpuscles in different stages of digestion. Immediately after the taking in of the blood by the Planarian all the red blood corpuscles are ingested by the epithelial cells of the intestine. Connected with the wall by slender stalks, these elements appear as large amoeboid cells whose free end projecting into the lumen of the intestine sends out protoplasmic processes which seize the red blood corpuscles and convey them into the interior of the cell. This goes on very rapidly, and in a very short time all the red corpuscles are found within the epithelial cells. As a result of the increase in volume of these cellular elements the intestinal cavity is completely occluded.

Once inside the cells of the intestine the red blood corpuscles exhibit changes which are readily followed under the microscope. It is better still to feed the Planarians with the blood of those lower Vertebrates whose red corpuscles are nucleated. In my researches I have used the blood of the goose. The red blood corpuscles of this bird, when ingested by the epithelial cells of the intestine of Planarians, are usually collected into compact groups (fig. 7), only a few remaining isolated. The majority of these red corpuscles soon lose their normal appearance and contour; they become rounded and fused together, but the nucleus and the haemoglobin enable us to recognise them without any difficulty. Later the red colouring matter begins to diffuse into the digestive vacuoles which form around the corpuscles. These corpuscles empty themselves, retaining their nuclei and capsules, which shrivel more and more. The nucleus also undergoes almost complete digestion, its membranous layer alone persisting (fig. 8). Even several days after the digestion of the blood has begun one can still find debris of perfectly recognisable red corpuscles, but the red colour has been replaced by a more or less pronounced brown tint. In the last stage of the digestive process, as the red corpuscles disappear, the protoplasm of the intestinal cells becomes filled with round vacuoles, containing brown irregular concretions—excreta—which are expelled into the intestinal cavity.

This slow digestion of a substance usually so easily assimilable as blood takes place entirely within the epithelial cells of the intestine. Continuous microscopical observation demonstrates most clearly the complete absence of any extracellular digestion of the blood corpuscles in the intestinal content.

When goose’s blood mixed with blue litmus powder is given to Planarians, the coloured grains may be found some hours afterwards inside the epithelial cells of the intestine, but only a few of the blue litmus granules change colour, taking on a light violet tinge; the great majority retain their blue coloration. It might be concluded from this that in Planarians intracellular digestion is effected in a neutral or nearly neutral medium. If, however, the preparations of intestinal cells gorged with goose’s blood are treated with a 1% solution of neutral red, we at once notice that the red corpuscles and the vacuoles which contain them are stained bright red, assuming a tint similar to that given with picrocarmine staining (fig. 9). This colour reaction indicates, according to our researches on neutral red, an acid reaction, more feeble, however, than that met with in Paramaecium and many other Protozoa.

Macerations of Planarians in normal saline solution to which has been added a small quantity of the red corpuscles of the goose’s blood exhibit in vitro a very distinct solvent action on these corpuscles, which become rounded and lose their haemoglobin, this latter diffusing into the surrounding fluid, and at the close of the experiment there remain simply the membranes and the nuclei of the corpuscles.

The study of these Planarians shows us, then, that the food of these animals undergoes exclusively intracellular digestion in a feebly acid medium and by means of a soluble ferment, and it furnishes us with proof that typical intracellular digestion is essentially a chemical process due to the intervention of enzymes. Now there can be no question, here, of a protoplasmic action proper, but the branched digestive canal, so intimately associated with the parenchyma, cannot be completely isolated from the rest of the Planarian, and it is impossible to study in vitro its digestive action apart from other tissues. To attain this end we must turn to animals of larger size and those in which the digestive organs can be isolated more easily. In the Coelenterata intracellular digestion is general. Many of them are so transparent that they can be examined in vivo. It is easy to observe that the particles of food are seized by amoeboid processes of the entodermic cells and that they pass into the substance of these elements there to be digested. For the systematic study of the digestive phenomena, however, it is not sufficient merely to examine all that takes place in the living animal. Experiment in vitro is also necessary. For this purpose the Actinians or sea-anemones offer us really excellent material. As these animals are very common in all our seas and are easily kept alive for long periods in aquaria, they have been used for various researches, among others for the study of the process of digestion.

The Actinians are easily fed in captivity; they devour morsels of flesh, of shrimps, of mollusca and other marine animals with avidity. The ingenious English observers Couch and G. H. Lewes^[77] long ago demonstrated that morsels of food when introduced enclosed in perforated quills or wrapped in test paper or gutta percha silk and swallowed by the anemones were afterwards ejected surrounded by mucus but with no trace of digestion. Having failed in their search for digestive juices in the large gastric or coelenteric cavity of the Actinians, Lewes concluded that digestion in these animals is effected in a purely mechanical fashion. The greatly developed muscles of the Actinians were supposed to squeeze the food and extract its fluid which is then absorbed by the walls of the general cavity. It was not until very much later that the problem of digestion in the Actinians could be resolved in any accurate and definitive fashion. More than twenty years ago I demonstrated^[78] that the digestion in these polyps is intracellular. In order that a clear conception of this phenomenon may be obtained it may be useful to recall in a few words the fundamental features of the organisation of Actinians. They are cylindrical bodies, sometimes as large as the fist, attached by their base to stones, shells, or other submarine objects, and furnished at their free extremity with one or more series of tentacles. In the middle of this extremity is an elongated opening, the mouth, which leads into a spacious sac, often spoken of as the stomach. It is, however, only a kind of oesophagus, through which the food passes into the large coelenteric cavity which is divided by septa into numerous compartments lined by the entodermic epithelium. These septa give origin to many very long and tortuous filaments, spoken of as mesenterial filaments from their resemblance, a purely superficial one, to the mesentery of higher animals (fig. 10). When the Actinian is hungry it protrudes its tentacles in order to seize marine animals, which it conducts to its mouth. The lips and the oesophagus are used to estimate the quality of the capture, and if it is found unsuitable the anemone rejects it, first surrounding it with a layer of mucus. If however the food is found to be suitable, the Actinian retains it in its large cavity and throws around it a multitude of its mesenterial filaments. These penetrate it in all directions, and as their epithelial cells are capable of sending out amoeboid processes they seize and ingest the particles, which immediately enter the protoplasmic content. This work is done with such precision and nicety that the sea-anemone is able to extract the contents of a shrimp from the carapace, which latter alone it rejects.

The epithelium of the mesenterial filaments is therefore the organ of digestion in the Actinians. The nutritive parts of their prey pass into the amoeboid epithelial cells and there undergo a purely intracellular digestion. If we add to the shrimp-muscle or other food a little carmine or blue litmus powder, the mesenterial filaments ingest it also and become pigmented. After eating carmine they assume a very brilliant rose colour (fig. 11); blue litmus colours them rose violet. This change of colour in the interior of the cells of the filaments indicates a decidedly acid reaction of their contents^[79]. When one adds to the mesenterial filaments which are carrying on the process of digestion a drop of a 1% solution of neutral red they assume various shades of red (fig. 12).

This intracellular digestion in the Actinians has been confirmed by several observers, amongst whom may be cited Chapeaux^[80] and Bjelooussoff^[81]. It has often been asserted, however, that, along with a digestion in the interior of the cells of the mesenterial filaments, there is, in the Actinians, a secretion in the coelenteric cavity of their body of fluids which digest nutritive matter by means of a soluble ferment. A ferment similar to trypsin has been extracted from Actinians by Léon Frédéricq and Krukenberg. But, in presence of contradictory assertions, it remained undecided whether, in the enzymatic digestion, this ferment does its work in the fluid of the coelenteric cavity or whether it represents the active factor in intracellular digestion.

With the object of definitely elucidating a problem of such general importance, Mesnil, the superintendent of my laboratory, has been good enough to carry out a fresh series of experiments on the digestion of the Actinians and has studied this process not only in animals kept in captivity in aquaria but also in Actinians living under natural conditions in the sea^[82].

As intracellular digestion is of interest to us specially in connection with the resorption of formed elements in the tissues and cavities of animals, Mesnil directed his attention to the digestion of the red corpuscles of the blood. He made use of the red corpuscles of several species of Vertebrata, but he made a special study of the digestion of nucleated red blood corpuscles. These corpuscles are very delicate, and may even undergo a certain degree of maceration in ordinary sea water. In spite of this these red corpuscles are not digested in the coelenteric cavity of the Actinians but, once ingested by the entodermic cells of the mesenterial filaments, they are completely dissolved by the intracellular digestion. Mesnil also observed that fibrin is not digested except in the cells of the filaments. The facts cited by Chapeaux in favour of an extracellular digestion in the fluid of the coelenteric cavity in no way support his hypothesis, and reduce themselves, according to Mesnil, to a digestion by the diastase of blood itself fixed by the fibrin, after the bleeding, at the moment of the formation of the clot.

For a certain period the red corpuscles may be met with inside the cells of the mesenterial filaments. They are ingested in their normal state—oval red corpuscles with a nucleus. As several hours are required for the ingestion, it is evident that the fluid of the coelenteric cavity has been incapable of attacking the red corpuscles. In the protoplasm of the entodermic cells the red corpuscles become rounded, their walls become permeable, and the haemoglobin begins to diffuse from them. It passes first into the vacuoles of the digestive cells and is then, in part, ejected into the general body cavity. The haemoglobin is transformed into a green substance which reminds one of biliary pigment. The membranes and nuclei of the red corpuscles are also digested and ultimately disappear completely.

The digestive cells of the entoderm ingest not only blood corpuscles or fibrin, but also fragments of muscular fibre and particles of carmine and litmus. These latter, as already stated, indicate a marked acid reaction.

In the Actinians, then, the mesenterial filaments, or rather their entodermic portion, represent the real organ of intracellular digestion. There are indeed other regions of the entoderm which also carry on this function, but in an insignificant degree as compared with the mesenterial filaments which are capable, however, not only of ingesting and digesting solid substances, but also of absorbing solutions. Mesnil has demonstrated this by injecting soluble colouring matters, such as eosin, carminate of ammonia, etc., into Actinians. These solutions, although in great part absorbed by the digestive cells of the mesenterial filaments, can, however, also be retained by other elements, amongst others, the cells of the ectoderm.

As the digestion of the food-particles goes on within the entodermic cells of the mesenterial filaments and as these organs can easily be isolated from the rest of the Actinian, Mesnil was able to study with great precision and care the phenomena of digestion outside the organism. With this object he prepared extracts of the filaments in sea water and studied their action on various nutritive substances. He confirmed the discovery of a soluble ferment made by Léon Frédéricq and demonstrated that it is capable of digesting albuminoid substances (fibrin, coagulated albumen) in media which are neutral, slightly alkaline or weakly acid. In this respect the actino-diastase (the name given by Mesnil to the soluble ferment of the Actinians) approaches most nearly to papain. On the other hand, it is distinguished by its greater sensitiveness to an excess of acid and also by its more powerful action on coagulated albumen.

The actino-diastase acts vigorously at any temperature between 15° and 20° C., but the optimum temperature for its digestive action is between 36° and 45° C. Higher temperatures weaken the diastatic power, and heating to 55–60° C. inhibits it completely. Among the products of the digestion of albuminoids by actino-diastase, Mesnil, like his predecessors, found not only a notable quantity of peptone but also products of the disintegration of the albuminoid molecule, such as tyrosin and proteino-chromogen. Consequently actino-diastase resembles Mouton’s amoebo-diastase in certain respects.

The nucleated red blood corpuscles of the lower Vertebrata are very convenient objects on which to observe the process of intracellular digestion within the cells of the mesenterial filaments. Mesnil has also studied them in vitro under the influence of actino-diastase. Under these conditions the phenomena of digestion recall very clearly those that have been observed within the digestive cells. The oval red corpuscles of the fowl and goose become spherical as a result of the solvent action on their membrane, and the haemoglobin diffuses into the fluid. The membranes and the nuclei of the corpuscles are, however, little altered and may be recognised under the microscope. The difference between this and digestion within the cells reduces itself to a more feeble digestive action of the aqueous extract. It is evident that the preparation of this extract is only capable of bringing into prominence a certain proportion of the actino-diastase contained in the entodermic cells of the filaments.

Mesnil has fed the same Actinians with repeated doses of blood with a view to make out whether the cells, under these conditions, acquire any special aptitude for the production of the actino-diastase. Notwithstanding numerous attempts, he could never assure himself that this takes place; the rapidity with which the red corpuscles were dissolved by the extract of the mesenterial filaments was the same whether this was prepared from Actinians that had been several times fed on blood or from those that had received none at all.

From what I have just described no doubt can exist that intracellular digestion is not a “protoplasmic” process essentially different from that which is brought about by the digestive juices secreted in the intestinal canal. In both cases we have a diastatic action, due to soluble ferments, produced by living elements. In intracellular digestion, however, the diastases carry on digestion in the interior of the cells, principally in the vacuoles, whilst in extracellular digestion this process goes on outside the cells, in the lumen of the gastro-intestinal canal.

It cannot be doubted that, in the animal scale, intracellular digestion represents an earlier and primitive condition for the solution of the food substances. This follows from the fact that it is widely distributed amongst the lowest animals, such as the Protozoa, Sponges, Coelenterata and Turbellaria. Intracellular digestion only gives way step by step to digestion by secreted juices. The higher Invertebrata furnish us with conclusive testimony on this point. Thus, among the gasteropod Mollusca, there are some which exhibit the two modes of digestion in the same animal. In Phyllirhoë, a beautiful mollusk, without a shell and quite transparent, which floats on the surface of the sea, the food can be seen passing into the cavity of the digestive canal, where it undergoes a preliminary digestion by secreted juices; the result is a magma of small solid particles which are at once seized by the amoeboid epithelium of the coecal appendages, two on each side of the body. Intracellular digestion then completes the process and ends by dissolving the nutritive substances and reducing them to their final stage previous to absorption. On adding to the food some particles of carmine these may be found along with the digestible particles in the interior of the epithelial cells of the coeca.