THE AMERICAN JOURNAL OF CANCER. A Continuation of The Journal of Cancer Research VOLUME XXXVI I I APRIL, 1940 NUMBER 4

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1 THE AMERICAN JOURNAL OF CANCER A Continuation of The Journal of Cancer Research VOLUME XXXVI I I APRIL, 1940 NUMBER 4 THE HORMONAL AETIOLOGY OF BREAST CANCER' W. CRAMER The experimental production of mammary cancer by oestrogenic hormones is one of the landmarks in the investigation of cancer. This is true not merely because these hormones represent another carcinogenic substance, or group of substances, nor on account of the fact that by this means cancer has been induced experimentally in another organ than the skin or the connective tissue. The importance of this discovery lies in the fact that the carcinogenic agent in this instance is a substance normally produced in the body and possessing definite physiological functions-in fact, a hormone. This adds to our knowledge of the aetiology of cancer a new conception, namely, that for at least one organ, the mamma, all the aetiological factors concerned in carcinogenesis may be normally present in the organism. This conclusion may at first appear to be a depressing one, but it has this compensation that it enables us to understand how cancer can arise in organs even less accessible than the breast to external carcinogenic stimuli, such as the thymus and the prostate. In these two organs we have, in fact, occasionally obtained the development of malignancy by prolonged oestrinisation. This new conception differs in many respects from that which has been built up during the last twenty years as the result of numerous experimental investigations on skin cancer. Up to now the process of experimental carcinogenesis has been studied almost exclusively with the skin as the test tissue, and the assumption has been made that the aetiology of cutaneous cancer is representative of the aetiology of cancer in all organs. There are, it is true, essential similarities between the action of the skin carcinogens and the mamma carcinogens, i.e. the oestrogenic hormones. There are also dissimilarities. Outstanding similarities are as follows: (1) There is a long period of induction. (2) Cancer arises in a sharply circumscribed area of the organ or tissue exposed to the action of the carcinogen. (3) The action of the carcinogen is conditioned by a factor of susceptibility residin'g within the animal. (4) Skin carcinogens and oestrogens produce epithelial proliferation in the skin 1 Read at the Third International Cancer Congress, Atlantic City, New Jersey, Sept. 13,

2 464 W. CRAMER and mamma respectively, and it is on the basis of this epithelial proliferation that cancer may arise. There are two notable dissimilarities. (1) The skin carcinogens act locally, at the site of application, but they induce cancer also in at least one remote organ, the lung. The oestrogens do not induce cancer at their site of formation or of application but in several organs on which they act functionally: most frequently the mamma, less frequently the uterus, prostate, and thymus. (2) Skin carcinogens are foreign to the physiological economy of the body. It has been suggested as a hypothesis that cancer may arise in the skin as the result of the formation of abnormal metabolites by epithelial cells placed under abnormal conditions, and that these abnormal metabolites possess carcinogenic properties. This has not yet been demonstrated. But even if it be true, the emphasis is on the word abnormal. The oestrogens are normal metabolites-i.e. formed in the body, with physiological hormonal functions. The problem of carcinogenesis in the mamma, at least that type of carcinogenesis induced by oestrogenic hormones, is therefore bound up with the physiological functions of these hormones. We do not know whether, after having induced the initial proliferation of normal mammary epithelium, the oestrogenic hormones transform normal mammary cells into malignant ones by a direct action, or whether an altogether different process supervenes, such as a biochemical change, a virus, or a mutation. But neither can we answer this question for the skin carcinogens. In fact the recent work of Wolbach and of Orr presents evidence that the skin carcinogens do not act directly on the cells. We do know that the oestrogenic hormones do not induce cancer in the mamma unless they have first produced their physiological effect on this gland. This is clearly evident when oestrogens are applied to male animals. There they first produce the development of normal mammary tissue, in one circumscribed area of which the malignant change subsequently occurs. Even if the oestrogens have a direct carcinogenic' effect, it does not manifest itself until they have produced their physiological effect on the mamma. That physiological effect is, like other hormonal effects, subject to control. We have thus, perhaps for the first time, the hopeful indication that a prophylaxis of mammary cancer by medical treatment is a reasonable' possibility. That at any rate is the reasoning which guided our investigations carried out during the last four years in collaboration with Dr. E. S. Horning. In this paper the results which we have obtained and the provisional conclusions at which we have arrived are briefly summarised. It is difficult at first to adapt the mind to the conception that a hormone can have carcinogenic properties. For if this were so it would be expected, since the carcinogen is always present in the body and always acting, that cancer would invariably arise spontaneously in the organ for which it is carcinogenic, as a physiological phenomenon. It has indeed been possible by close inbreeding to obtain strains of mice in which 70 per cent, 80 per cent, and even 90 per cent of all the females develop spontaneous mammary cancer, as if it were a physiological phenomenon. On the other hand, strains have been obtained in which only 2 per cent of the females, or even none at all, develop mammary cancer spontaneously. Nevertheless, the carcinogenic property of oestrogens

3 THE HORMONAL AETIOLOGY OF BREAST CANCER 46 5 is based on evidence which is incontrovertible. For if oestrogens are applied to animals belonging to these various strains mammary cancer will develop in all of them, in males as well as in females. The only differences are that it will develop more rapidly with a smaller total dose and in greater numbers in the high-cancer than in the low-cancer strains. Dr. Bonser and her collaborators have shown that oestrogens will induce mammary cancer even in strains in which it does not appear spontaneously. Dr. Horning and the writer have demonstrated that it is possible to induce mammary cancer-sometimes very rapidly-in mice which are not inbred and in which the normal incidence is low. Similar results have been obtained by Burrows and others in non-inbred mice, while Geschickter in the United States has reported the induction of cancer by oestrogenic hormones in the mamma of rats. Now in rats spontaneous malignant tumours of the mamma are very rare, and in the strain used by Geschickter no mammary cancer had appeared spontaneously in 2,000 animals which had been kept under observation. After the application of massive doses of oestrogenic hormones, however, mammary cancer developed in 25 out of 86 young rats. It is clear, therefore, that a normal metabolite with hormonal functions has carcinogenic properties for the mamma in the same sense that tar, benzpyrene, and other skin carcinogens have for the skin. But its carcinogenic effects are equally clearly subject to limitations. How can we account for this? The most obvious possibility is that mammary cancer appears as a result of a hypersecretion of this hormone. But observations on high-cancer strains failed to give any evidence of this. We must assume, therefore, that the carcinogenic effect of oestrogens is limited by factors inherent in the animal. We call these factors ( susceptibility. They also limit the effects of.the skin carcinogens, a fact to which the writer called attention twelve years ago, before the carcinogenic property of oestrogens had been demonstrated. It was shown at that time that a carcinogenic stimulus which will produce skin cancer in one individual will leave another individual intact. As the carcinogenic stimulus is increased, more and &ore animals will develop skin cancer. Recently, several workers-andervont, Bonser, and Dobrovolskaia-Zavadskaia-using inbred strains of mice have again demonstrated experimentally the importance of this factor of susceptibility for the aetiology of skin cancer. Zavadskaia calls this factor the great unknown-the terrain. For the aetiology of spontaneous skin cancer the presence of the carcinogenic agent-a substance foreign to the physiological economy of the organism -is of decisive importance. For the aetiology of spontaneous mammary cancer, where the carcinogenic agent is a hormone normally formed by the body and always present, the factor of susceptibility is of decisive aetiological importance. This difference affords an explanation for the fact that it has, so far, not been possible to obtain by inbreeding strains of mice with a high spontaneous incidence of skin cancer or strains completely resistant to the action of agents carcinogenic for the skin, while there do exist inbred strains where the incidence of spontaneous cancer of the mamma is either very high or very low together with other strains where mammary cancer does not appear spontaneously and in which it can be induced only with great difficulty and by very massive doses of oestrogens. In strains in which the susceptibility is

4 466 W. CRAMER high the mamma will develop cancer spontaneously if it has been exposed to doses of the ovarian hormone so small that they lie within physiological limits. This was demonstrated by A. E. C. Lathrop and Leo Loeb twenty years ago by removing the ovaries of mice at different ages. Their results, which were confirmed later by Cori, showed that ovariectomy at six months of age had no effect on the subsequent mammary cancer incidence, but when it was carried out at earlier ages the development of mammary cancer was either delayed or prevented. Cori concluded from his experiments with a high-cancer strain, in which the first mammary cancer appeared at a very early age, that in this strain exposure of the mamma to from 5 to 20 oestrous cycles was sufficient to bring about the development of cancer after an interval of time. In other words in such a strain the mamma passes into a precancerous stage at a fairly early age, but the actual appearance of malignancy may not take place until several months have elapsed. When, however, susceptibility is low the amount of oestrogens necessary to induce cancer is large and lies quite without physiological limits. But with such large doses cancer can be induced in animals in which it does not appear spontaneously. The outstanding importance of the constitutional factor of susceptibility distinguishes the aetiology of mammary cancer from that of skin cancer, and this distinction enables us for the first time to understand certain features concerning the incidence of cancer in man in different groups of organs. There are certain organs for which the cancer incidence is governed largely by an inherited constitutional factor. The outstanding organs in this group are: the mamma and uterus in women, and probably the prostate in man. There is another group of organs in which the incidence is governed either by the occupation or by social conditions-so that we have what may be called social cancer. These organs are those exposed to influences and stimuli coming from without; they include the skin and the digestive tract from the lips to the stomach inclusive. In view of the aetiological importance of the susceptibility factor for mammary cancer it becomes necessary to inquire into the rtature of this factor. We know that it exists for both the skin and mamma, but about its nature little is known. We do not even know whether susceptibility resides partly, or at all, within the tissue in which cancer develops. We know, especially from the work of Kreyberg.and of Dobrovolskaia-Zavadskaia as well as the writer s own experiments, that susceptibility for skin cancer is different from the susceptibility for mammary cancer. The skin of mice from a strain with a high incidence of breast cancer does not respond more readily to agents carcinogenic for the skin. For the mamma the experiments carried out by Homing and the writer give some evidence that susceptibility is dependent on the functional activity of the endocrine system, that is to say on a factor outside the mamma. It was found that the action of oestrogenic hormones is not limited to the female sex organs, but extends to the whole endocrine system, the organs most prominently affected being the pituitary body and the adrenal glands. In the pituitary the changes affect almost exclusively the anterior part. Changes visible to the naked eye are a massive enlargement and intense congestion, which may lead to haemorrhage into the gland. Even more important are

5 THE HORMONAL AETIOLOGY OF BREAST CANCER 467 the histological alterations. The acidophile cells are greatly diminished in number and may be so few that the enlarged glandular mass consists almost entirely of chromophobe cells. This profound alteration has been interpreted by us as indicating a continued stimulation of the anterior lobe, which, if prolonged, leads to exhaustion of the acidophile cells believed to be responsible for the secretion of one of the specific hormones of the posterior pituitary. In the adrenals it was found that in mice the prolonged application of oestrogens always produces a specific degenerative lesion, even in strains with a low incidence of spontaneous mammary cancer. A similar lesion develops spontaneously, i.e. without administration of oestrogens, in the adrenals of two high-cancer strains of mice which we had an opportunity to examine, but not in the adrenals of three strains in which mammary cancer rarely develops spontaneously. Similar results were obtained by Dobrovolskai'a-Zavadskai'a, who has confirmed the occurrence of this degeneration in the high-cancer strain R I11 and its absence in five other cancer-free inbred strains. There is, however, a significant difference between this piocess as it develops spontaneously in a high-cancer strain and the process induced by oestrogens in low-cancer strains. In the latter it begins always immediately outside the medulla in the zona reticularis and remains mainly confined there as it progresses. The spontaneous process of degeneration, on the other hand, as seen in high-cancer strains affects chiefly, though not exclusively, the medulla of the adrenal. Reference will be made presently to the significance of this fact, that under different conditions the two histogenetically and functionally distinct parts of the adrenal are affected. At present it may be pointed out that the evidence shows clearly that the spontaneous development of brown degeneration in the medulla is not due to inbreeding per se, nor is it the result of the presence of a tumour in the mamma, since it sets in in all the mice belonging to these strains whether or not a tumour has developed. The regular occurrence in the two highcancer strains of this process at an early age, and before tumours have arisen, shows that in these two strains the inherited susceptibility to the development of mammary cancer is associated with the destruction of large parts of the adrenal medulla by a spontaneous process of degeneration. Another line of investigation concerning the part played by the adrenal in the aetiology of mammary cancer was pursued by studying the action of oestrogens in adrenalectomised mice (Cramer and Horning 1939). It was found that following adrenalectomy the general physiological effects of oestrogens, such as arrest of growth, emaciation and testicular atrophy, are diminished. Development of the mamma in response to oestrogens was also impaired, and carcinogenesis was either delayed or prevented. But the presence of even a minute amount of cortical tissue in the form of a cortical adenoma was sufficient to restore completely the sensitivity to oestrogens, both as regards their physiological and their carcinogenic effects. This means that the presence of a cortical hormone of the adrenal is necessary for the full 'development of the action of oestrogens. This cortical hormone acts therefore synergistically with the oestrogens. This conclusion is in agreement with the recent observations of G. Woolley, E. Fekete, and C. C. Little, who recorded the remarkable fact that in mice ovariectomised at birth the adrenal cortex can take over the hormonal functions of the ovary in so far as it can bring about a normal develop-

6 468 W. CRAMER ment of the uterus and of the mamma. In a high-cancer strain it can even lead to carcinogenesis in the mamma. The fact is significant that, in spite of the removal of the bulk of the cortical tissue, the presence of a minute amount of this tissue in the form of an adenoma no larger than a pin s head is sufficient to reestablish the sensitivity of the organism to the action of oestrogens. In the adrenalectomised animals with a cortical adenoma the medullary tissue is absent. The fact that in such an animal a minute amount of cortical tissue is sufficient to restore the sensitivity to oestrogens suggests that the adrenal medulla also plays a part in conditioning the action of oestrogens, but in a sense opposed to that of the cortex, that is to say, as an antagonist to the oestrogens. This interpretation falls into line with the conclusions arrived at from a study of the brown degeneration in the adrenals of inbred mice belonging to high-cancer and low-cancer strains, where it was found that a high susceptibility of the strain as a whole was associated with a destruction at an early age of the adrenal medulla by a spontaneous process of degeneration and a consequent weakening of a hormonal factor antagonistic to the oestrogens. The dysfunction of the adrenal medulla produced by this process of degeneration thus offers itself as an explanation of a high susceptibility to mammary cancer of these two strains. The same effect, an antagonism to oestrogens, could be demonstrated for a hormone from the anterior pituitary, the so-called thyrotropic hormone. The existence of these relationships between oestrogens on the one hand, and the adrenals and pituitary hormones on the other, can be recognized in the living animal. When oestrinised mice of three groups are compared, namely, (1) intact mice, (2) adrenalectomised mice, and (3) mice treated with the pituitary hormone, thyrotropin, it is possible to distinguish readily the small, emaciated mice subjected to the effects of oestrin alone, from the larger, well nourished oestrinised mice which have been either adrenalectomised or thyrotropinised. The physiological effects of the ovarian oestrogen are, therefore, conditioned by at least two other hormones, one from the anterior pituitary, which antagonises the oestrogen, and one from the adrenal cortex, which enhances the action of the oestrogen. The adrenal medulla contains probably a third conditioning factor, which, like the thyrotropic hormone, inhibits the action of oestrogens. This relationship extends to the physiological action on the mammary gland, which responds to the oestrogen by epithelial proliferation less vigorously if the animal has been either completely adrenalectomised or thyrotropinised, and also, within limits, to the development of cancer in the mamma. In experiments by Horning and the writer the spontaneous development of mammary cancer could be prevented in the females of a high-cancer strain by thyrotropinising the animals. Our object was to protect the mammary epithelium against.the action of the ovarian hormone by daily injections of the antagonistic pituitary hormone. Since Cori had shown that even a very short exposure of the mamma to the ovarian oestrogen was sufficient in a highcancer strain to elicit cancer of the mamma, which might not appear till several months after the exposure, we began our treatment early-at the age of two months. This condition was not observed in the experiments of Lacassagne.

7 THE HORMONAL AETIOLOGY OF BREAST CANCER 369 The objections which he has raised against our interpretation appear to be due to a misunderstanding of our conception. SUMMARY We can now summarise the results of our investigations as follows: A high susceptibility to mammary cancer is associated in two strains of mice with a process of degeneration in the adrenal medulla. It can be diminished either by removing the supply of the adrenal cortical hormone or by applying an excess of an anterior pituitary hormone. From this the following conclusions can be drawn concerning the susceptibility to mammary cancer. (1) It is not a fixed and unalterable quality residing within the organism, but can be modified. (2) It does not reside entirely-if at all-within the mamma. (3) It is conditioned partly by a disturbance of the endocrine balance. If we visualise this endocrine balance literally, as shown in Fig. 1 we have on CANCER MAMMA - INVOLUTlON Adrenal HMcdulla FIG. 1 one side of the scale the oestrogenic ovarian hormone acting as the carcinogenic agent and the synergic adreno-cortical hormone without which the oestrogen cannot exert its full activity. On the other side are the two hormones antagonising the effects of the ovarian oestrogen, namely, the thyrotropic hormone of the anterior pituitary and a hormone from the adrenal medulla. The effect on the mamma, as the scales move up or down, is indicated by the pointer of the balance. The gap between a fully developed mamma and a cancerous mamma indicates that the process of carcinogenesis is not simply a continuation of the process of development. The two are discontinuous. A new event-as yet unknown to us-takes place. Whether the hormonal factors play a part in that event or not remains an open question. As the diagram shows, the development of cancer in the mamma can be brought about by disturbing the endocrine balance in a variety of ways. In one the carcinogenic agent-the ovarian oestrogenic hormoneis increased. This is the method used experimentally, and when this is done the organism

8 470 W. CRAMER tries to restore the endocrine balance by a degenerative process in the adrenal cortex which tends to diminish the synergic hormone of the cortex, and by an increased functional activity of the anterior part of the pituitary which secretes the antagonistic hormone. This increased activity manifests itself by hyperaemia of the anterior part and by a hyperplasia, and eventually leads to an exhaustion of the acidophile cells. From this point of view the degenerative changes in the adrenal cortex and the hyperplastic changes in the pituitary are to be interpreted as an attempt on the part of the body to prevent an excessive development of the mamma and a subsequent development of cancer in that organ. It is not a part of our conception that they are essential for carcinogenesis in the mamma or even favour it. If the carcinogenic agent-the oestrogen-is not increased, cancer may develop in the mamma as a result of a sensitisation of the organism to the action of the ovarian hormone by a decrease in its antagonists. This is brought about in the two inbred strains examined by us by a spontaneous degenerative process in the adrenal medulla. Or a pathological condition in the anterior part of the pituitary may bring about a dysfunction of the acidophile cells and thus weaken an antagonistic hormone. For the human subject the existence of such pathological conditions of the pituitary in association with mammary cancer has been recorded in the literature. Ex hypothesi an increased functional activity of the adrenal cortex should also increase the susceptibility of the mamma to the action of oestrogens and thus tend to favour the development of cancer. Such a hyperfunction of the cortex may be brought about by the existence of cortical adenomata. We have not yet followed up this line of investigation, but the recent work of Woolley, Fekete, and Little on the mouse, referred to above, of Bagg on the rat, and of Greene on the rabbit, lends support to the conception of an endocrine imbalance, especially of the ovary, pituitary, and adrenal, as a determining.aetiological factor in mammary carcinogenesis. Conversely a resistance to the development of cancer in the mamma-in other words a prevention of mammary cancer-an also be brought about in a variety of ways. The carcinogenic agent may be diminished by ovariectomy -the effect of this procedure in delaying or preventing mammary cancer having been demonstrated by Leo Loeb and by Cori. We have shown that the same effect can be produced by diminishing the susceptibility either through removal of the synergistic adrenal hormone or by applying an excess of antagonistic anterior pituitary hormone. The finding of Nathanson and Andervont and of Lacassagne that the male sex hormone, testosterone, is also antagonistic to the effects of oestrogens does not preclude a similar action by a pituitary hormone, but adds another hormonal factor to those identified by us as determining the development of the mamma and the subsequent carcinogenesis in that organ. In the diagram the male hormone should, therefore, be placed on the same scale of the balance as the two other antagonists. The conditions determining susceptibility of the " terrain " are, as was said earlier in this paper, of decisive importance for the aetiology of cancer of such organs as the mamma for which a substance formed normally in the body and always present can act as a carcinogen. We present our conception of an endocrine imbalance as an aetiological factor as a first attempt to penetrate

9 THE HORMONAL AETIOLOCY OF BREAST CANCER 47 1 into this '' great unknown )' and to make it accessible to an experimental analysis. It is a working hypothesis intended to correlate a number of new facts which have come to light, and will probably require to be extended and modified as further new facts come to light. We have already obtained some evidence of the existence of other factors and we have also found that oestrogenic hormones can be carcinogenic for organs other than the mamma (Cramer and Horning 1939). In the uterus of oestrinised animals cancerous and precancerous conditions have been described by several American workers (Gardner, Allen, Smith and Strong, Leo Loeb and his collaborators). We have obtained in two oestrinised mice malignant changes in the uterus, and Korenchevsky has produced conditions resembling precancerous lesions by oestrogens in combination with other sex hormones. We have also observed in our material a mouse with a carcinoma of the prostate and another mouse.with an epithelial tumour of the testis, which we believe to be derived from the interstitial cells. Perhaps the most remarkable, because the most unexpected, findings have been tumours of the thymus. In our material we have met with seven such tumours of which two were undoubtedly malignant, both with metastases in the kidney. None of the oestrinised mice with tumours in uterus, prostate, testis or thymus had developed a mammary cancer, and of those fairly numerous cases of uterine cancer in oestrinised mice which have been recorded in the literature, only very few are recorded to have been associated with a mammary tumour. It is not known at present what the factors are which switch over the carcinogenic action of oestrogens from the mamma to other organs. This will, no doubt, become evident as investigations proceed and is likely to throw new light on the actual process of carcinogenesis itself. The switch-over of the action of oestrogens from the mamma to the thymus and sometimes to the whole lymphoid system is surprising. Ordinarily, the prolonged administration of the oestrogens leads to an atrophy of these organs, especially of the thymus. This is associated with arrest of growth and emaciation, an association which appears to be a general one. But there are occasionally mice treated with oestrogens which continue to grow, get fat, and attain a weight above the normal. In such animals the thymus is persistent and the lymph nodes are larger than normal. Removal of the adrenals produces the same condition. This condition represents, perhaps, what was formerly known as status lymphaticus in man. From the point of view of human cancerology the conception of an endocrine imbalance as an aetiological factor in cancer of the mamma, and probably also of the uterus, should serve as a useful guide for clinical investigations. There is now a possibility of treating precancerous conditions of these two organs by appropriate hormones. The uterine cervix being an accessible organ in which precancerous conditions can be readily recognised seems to offer itself as a most likely object for such an investigation. The statistical work of Waaler and of Wassink has established the existence of a population of women with an inherited susceptibility to mammary cancer, and of yet another population with an inherited susceptibility to uterine cancer (for literature see W. Cramer 1937). It would be desirable to search for clinical features resulting from slight endocrinological abnormalities which characterise women belonging

10 472 W. CRAMER to these two groups. If it were possible to recognise such a state of susceptibility clinically, before the disease had declared itself, the endocrinological abnormalities on which such a susceptibility is based could be corrected and these two numerically important types of cancer might be prevented. REFERENCES BAGC, H. J., AND HAGOPIAN, F,: Am. J. Cancer 35: 175, BONSER, G. M., STICKLAND, L. H., AND CONNAL, K. I.: J. Path. & Bact. 45: 709, BURROWS, H.: Am. J. Cancer 24: 613, COPI, C. F.: J. Exper. Med. 45: 985, CRAMER, W.: Am. J. Cancer 30: 318, CRAMER, w., AND HORNINC, E. s.: Lancet 1: 247, 1056, 1936; 1: 72, 1938; 1: 192, 1939; J. Path. & Bact. 44: 633, 1937; Am. J. Cancer 37: 343, DOBROVOLSKAXA-ZAVADSKA~A, N., AND Z~PHIROFF, P. : Compt. rend. SOC. de biol. 128: 971, DOBROVOLSKA~A-ZAVADSKA~A, N., AND PEZZINI, Z. M.: Compt. rend. SOC. de biol. 131 : GARDNER, W. U., ALLEN, E., SMITH, G. M., AND STRONG, L. C.: J. A. M. A. 110: 1182, GESCHICXTER, C. F.: Science 89: 35, GREENE, H. S. N.: J. Exper. Med. 70: 147, LACASSAGNE, A.: Compt. rend. SOC. de biol. 130: 591, LACASSAGNE, A,, AND RAYNAUD, A.: Compt. rend. SOC. de biol. lsl: 586, LATHROP, A. E. C., AND LOEB, L.: J. Cancer Research 1: 1, NATHANSON, I. T., AND ANDERVONT, H. B.: Proc. SOC. Exper. Biol. & Med. 40: 421, ORR, J. W.: J. Path. & Bact. 46: 495, SUNTZEFF, V., BURNS, E. L., MOSKOP, M., AND LOEB, L.: Am. J. Cancer 32: 256, WOLBACH, S. B.: Am. J. Path. 13: 662, WOOLLEY, G., FEKETE, E., AND LITTLE, C. C.: Proc. Nat. Acad. Sc. 25: 277, 1939.