Document 8Vj7L1mwwv9r3GjmzbX0Yx5Yk

ses of intracranial tumors in the two countries. The mortality rates ranged from 3 to 5 per 100,000 populjlion. If this mortality estimate was correct in 1962 and if it is still valid in 1974, one can expect between 6,300 and 10,500 deaths from intracranial neoplastic disease this year in I his country based on a population of two hundred and ten million. The compilation of data of this kind is subject to innumerable inac curacies, as pointed out by KURLAND and his associates. They noted, for example, that the rate for deaths due to brain tumors in Japan was very low, being about half that in Western countries, and they felt that at least in part this low rate could be attributed to diagnostic and reporting artifacts. These investigators further reported the results of a 10 year survey among the population of Rochester, Minnesota. They found a prevalence ratio of intracranial neoplasms of 46 per 100,000 population and an incidence rate of about 19 per 100,000 per year. In this population, about 1 per cent of all deaths was due to primary intracranial tumors, which was about twice that reported in official mortality statistics. Other studies have been carried out in the United States that disclose rates that vary from 3.0 - 8.4/100,000 of ihc population (2, 4, 5), and in other parts of the world the incidence also varies considerably (6, 21 Based upon confirmed tumor diagnosis at biopsy or autopsy, it has been estimated that primary intracranial neoplasms constitute 2-3 per cent of all neoplasms to which man is heir (8). There exist pub lished accounts of the incidence of various expanding, space-occupying intracranial lesions, not all new-growths and not all primary neopLasms of the brain, that have been collected in different countries of the world. Some of these statistical reports have been presented in table form by this author (3), revealing wide discrepancies in the incidence of certain categories of tumors fiom country to country and even from series to series of cases collect' <i by different authors in the same country. Despite the reported variations in incidence of intracranial tumors in different series, all contributors aqreo that the gliomas rank first on a numerical basis. They constitute between 31 to 43 per cent of all intr as t pand ing be S' tumo tors low, Amon< lign. pies Next cytot incii drog. tial losut ser k the t tif i; that part; matoi THE 1 It IE ble it tuall prodt chemi benzp cytes the it f reqt neopl By fa gliot tumor man g phic, in pal Vascul neopla A modi the nt enume plast calle The s matou unequ Still cytic * v>rt all ty (ionvecl . Their I ' ' 1 l 5J " ...... . L j luatc xpect sease and i i**t -- ' . ij*u[ the a table .cidence _-n from same .ii. irs in l i i st of all intracranial masses, including metastatic tumors and granulomas such as tuberculomas. Simply eliminating the latter two categories of ex panding intracranial lesions from consideration has the effect of rais ing the glioma |.ii,irl',iKf' I o bf'i-wonn 40 and SO [.or cent. Thus it can be seen that I or numencaL conn ider.it ions alone this class of brain tumors is the most important (9). Hut there arc many additional fac tors, some subject to experimental probing, as will be discussed be low, that make it so important. Among the human gliomas, tins ant hoi lias found that the relatively ma lignant astrocytic tumor of the g 1 i ob las Ionia niultilorme variety occu pies first place numerically with an incidence of over 51 per cent (3). Next in frequency is the more slowly growing, hence more benign, astro cytoma (incidence of 24.5 per cent). The ependymoma is next with an incidence of slightly more than 6 per cent, followed by the oligoden droglioma (5.5 per cent). The polar spongioblastoma, confined essen tially to the pons and brain stem and occasionally to the corpus cal losum, was present in 3.4 per cent of the 1,633 glioma tumors of this series. Attention is drawn to the fact that 57 (3.4 per cent) of all the gliomas were mixed tumors: i.e., they contained more than one iden tifiable gliogenous component. Among these "mixed" gliomas were some that were I'nmposed of both I'ti.'ndymomatour. and n l i gndendrog 1 lomat.ous |.-1 i I :; , .111-1 .< ,|||. ` I li.ll 11,1,1 . II Id i I i . in. i I , i . I i < n `y 1 t >111.11 I HI*: ,i:. wi ' I I as a a rcomatous portions. THE EXPERIMENTALLY PRODUCED GLIOMAS It is fortunaLc that the experimentally produced gliomas often resem ble morphologically their human counterparts so closely as to be vir tually indistinguishable from them. Thus the cerebral astrocytoma, produced in mice by intracerebral implantation of compact pellets of chemical carcinogens such as methylcholanthrene, dibenzanthracene or benzpyrene (10), is a microcystic tumor composed of stellate astro cytes (Fig. 1) very much as are the human astrocytomas. In general, the more slowly growing tumors of astrocytic origin are produced less frequently with the aromatic hydrocarbons than are the more malignant neoplasms. By far the most frequent murine glioma produced experimentally is the glioblastoma multiforme (Types III and IV astrocytoma). It is this tumor, as already mentioned, that occupies first place among the hu man gliomas. Like the latter, the murine neoplasm is highly pleomor phic, with foci of necrosis and marginal spongioblasts usually arranged in palisades (Fig. 2). Multinucleatcd tumor giant cells are common. Vascular thromboses and hemorrhages arc also frequent features of this n.'op I asm. A modification of the murine glioblastoma multiforme exists in which the neoplasm, in addition to disclosing all the cytologic details just enumerated, is also characterized by numerous huge multinucleated neo plastic cells (Fig. 3). This is the tumor which in man is sometimes called "gigantocellular" or "monstrocellular" glioblastoma multiforme. The suggestion that the human tumor may be of mesodermal, hence sarco matous, origin is not valid for this tumour in the mouse, which is unequivocally a glioma as shown by special staining methods. Still another variation of the malignant form of glioma of the astro cytic variety seen in the corpus callosum, pons or brain stem of the 7 t' t'J- -J> 4. ILs s nd l *>s oj' nf'.ir.j-jbl j` n. 9 mouse is characterized by large interlacing bundles of bipolar and unipolar spongioblasts (Fig. 4). There are present occasional necrotic foci in some of these tumors and also a few giant colls with multiple nuclei. A considerable degree of cellular pleomorphism is always pres ent. In man, this tumor is designated by the term "polar spongioblas toma . " With pellets of carcinogen implanted in the subcortical white matter of the animal brain, there is often produced a glioma of distinctive cellular appearance. The rather large uniform tumor cells are compart mentalized into lobules, almost gland-like in appearance, by vascular ized connective tissue septa (Fig. 5). The individual neoplastic cells have clear cytoplasm between the chromatin-rich, round nuclei and the cellular membranes. The pale cytoplasm creates the appearance of "haloes" around the nuclei. This tumor is the easily recognized oligo dendroglioma. Less frequently than the similar tumor in man, this mu rine glioma has calcium salt deposits. I /*' i rj , . U L i <J<><11 nd r<nj / / < `trhi . L"h . h-i r` / r* ' . ' ` ' V ' ivith cytoplasm that imparts an >pp` /.mi,,-. Vascularized connective tissue sepia livia- the x 460 f L // mi. r- mud t. u. i ,* i' . ]` po n i nttc I <L a r halo l s . 'umur into 'obules. When the chemical carcinogen is implanted intraventricularly, there very often appears a tumor whose origin can be traced to the lining ependymal cells. Characteristically, the tumor cells form rosettes either around blood vessels or around acellular centers that are filled with delicate fibrils whose origin is from the neoplastic cells (Fig, 6). Electron microscopic study of these ependymomas discloses the same junctional complexes and cilia (or their precursors known as "basal bodies") that are diagnostic of the human tumor of this variety (VI, 1_2) . 10 m t >. * f Fig- ? Myx, the ependym, 1 t- }CJ- J- I I I ` oci M. !, 1 J) . It There is a variation of this tumor in man that is called a myxopapillary ependymoma, and precisely this mfirpholotpc variant is sometimes seen in the experimental animal (Fig. 7) . Its origin from the ventric ular ependyma is clear. It is composed of tumor cells arranged in pa pillae between which is deposited a proteinaceous, myxomatous, PASpositive substance. Still another variation of the ependymoma is seen in the experimental animal 'and is similar to the rare human mcduiioepithel ioma (1_3). This neoplasm has a tubular and papillary arrangement of its columnar cells (Fig. 8). In part it appears to be well differentiated, but some por tions of the tumor are quite primitive in appearance and contain numer ous cells in mitotic division. reason nogens may ac uced, differ In a f or the suited recogn cytes by the ' ij. .'i i papillary ntruatut'' a. r ... / intn.ir i, i: f ir frrm t ;h , t , CYTOGENESIS AND ETIOLOGY OF EXPERIMENTAL GLIOMAS The murine gliomas induced with hydrocarbons are derivatives of the three basic glial cell types of the adult animal brain: the astrocyte, the oligodendrocyte and the ependyma. It is emphasized that in 35 years of experimentation during which several thousand mouse brain tumors were produced, not a sinqle neoplasm developed whose origin could be traced to t In' lourth glial element nl (lie brain; namely, the micro glial cell. It is further emphasized that all the gliomas were invari ably induced in adult animals and that the tumorigenic process was started by chemical implantation in the brains of young adult animals (from 6 weeks to 6 months of age). This was done for the purpose of t? Fig. uihose appro The S of a a and by experi bellar glia ( toma, neuron cells plast of th The p comas on th fr -L.U- tlf.'S .1 10' ]>a- :ntai This oo Lis I'*'*"- reasonably excluding the possibility of the interaction of the carci nogens with immature or embryonic glia, or embryonic cell rests. It may account for I tic fact ll.it not a .tingle g. i ng l l ikj ] t oma was ever prod uced, presumably because uflei neuron:; reach maturity and are fully difforentLated they no longer are capable of neoplastic transformation, in a few instances when tumors developed in the cortical gray matter or the basal ganglions, a superficial appearance of gangliogliomas re sulted (Fig. 9). The malignant glial cells in such tumors were easily recognizable and mature neurons were found among them, but the neuro cytes were not neoplastic. They appeared merely to have been entrapped by the proliferating g1ia. Lho :'ucyte, 3'" years icrs 1 d be c i oi i. v a r i e/ i ' . ! . ' <1 I J * . /` -L. /. a. H'l i j'J`` r> h 1 i `t I < / '11 ! ' 7'/ * h 1 i;i rrw'h t> 1 p,< //,; / j 1 i i nil 1 `. . " i n 11./ ` / n y a , , * r>y tiKurunn Ij'y r / / 1-y 1 ` ! hr r/uiir;IL ry L u Lug ic`(l L - rpe , .`me M t a / LuinarS. X 680 The situation regarding the production o medulloblastomas is rather of a different order. These tumors, too, were produced with some ease and by chemical carcinogens in tne cercbella of adult mice (1_4), The experiments reveal that these tumors have their origin in the cere bellar cortex, evidently from the small granular cells and not from glia (Figs. 10, 11). This would make the medulloblastoma a neuroblas toma, and not a glioma at all. It would seem that in the process of neuronal differentiation in the cerebellar folia the small granular cells had not achieved the degree of maturation as to preclude neo plastic transformation, as is the case with the larger ganglion cells of the cerebellar cortex. The problem of the "mixed" glioma,, ,is well an of 1 he mixed gliomasartx.nias, seen bol h i u man and M.e iik.iim' .-im . I n be readily explained on the basis of the following consideration. The small carcinogenic 13 ! / j , I ii. Mi'ihi t` I nh I n: t \ *m.i. t"h . ' m. > r .<//:; *// /./.? 'i/i-,: ih>ii ' t-. r.> mb / . i > I I it I. I t` !<]'< r tn(' r <j r:n h l u r '//< /.'/> <'J' tin' J'ljur'1, .' H i" t T``l hn 1 ! I l>-l '< ` . fn /,/</ u.luLt *n</n.:i . pellets, which measure approximately 1 mm in diameter and are implanted in the brains for the purpose of producing the neoplasms, are many times larger than any of the cells in whose midst they are placed. In this way many different cells of the same type, and possibly also of different types, are stimulated by the carcinogenic agent to undergo neoplastic proliferation. Thus the simultaneous proliferation of cells of different types gives rise to the mixed tumors. Perhaps the marvel is that such tumors do not actually occur more frequently than they do, for "pure" tumors, i.e. those of one cell type, can evidently occur if only a single cell gives rise to the neoplasm or if only cells of the same and not other types are affected. A more detailed study of the mechanism of carcinogenesis with chemical agents than was afforded by light microscopy was made possible with the availability of the electron microscope. It had been shown that the induction of cerebral neoplasms with carcinogens followed the process of phagocytosis of the chemical crystals by those cells destined to become neoplastic (1_5). What ensued within the chemical-bear- 14 ure . and r very ges t ing c gestf the c app diamt presc to tl glior tumoi tumoi neopl confi. these I of anv logic] From < blastd folictumoi mesoc gesti siom and ( ulom. implantarc many lueodl In ,i 1 so of undergo . of cells I.,- marvel hi they do, occur if :s of the \ chemisslble *, shown i I' >w'h , !/1 i i- /-/././.' 1 "-'n t, It t . i, I y iri'l Lhf'r-t' ;`r'l`l i -it ly n-i 1 t`, ittm t It t'l-ry i-nHi- r i,Jw ' ht j't'jur't'i ,;,itn tli:r.,!r ,, ges L a rout-t L--. .c '.`v, it I` , ' I II It, r ft tty /,, r t g~ i r. i, a - J't i rm -I t(:l" ,i /-: i< -`I r / / t,i / i^r rit-:in ut rn ; l _ ; n tin? ; r'' ! rr-iriiifd lift -id io :: ug- ing cells that resulted in their neoplastic transformation was sug gested by several electron microscopic studies (!, Ur 18). Within the cytoplasm of the cells reactinq to the carcinogenic crystals there appeared uniform filamentous and sphcric.iL particles with an outer diameter of about 77 m'1 (Fig. 12). These intracellular particles were present only during the precancerous stages of the cellular response to the pellet implantation; they disappeared from the cells when a glioma could first be recognized microscopically. Efforts to produce tumors with these virions in saline suspensions that were free of whole tumor cells and of carcinogen resulted in the production of malignant neoplasms at the injection sites in 8 of 14 animals (8). Additional confirmatory evidence is still necessary for the establishment of these virions as agents in experimental g1iomatogenesis. No evidence of any kind is as yet available that would implicate viruses as etiologic agents in human gliomas. From time to time it was suggested in the past that the human glio blastoma multiforme was caused by an infectious agent (1_9) . It was the following considerations that were responsible for this view: 1, this tumor frequently has extensive necrosis; 2, it often has a partial mesodermal stroma; 3, there are vascular proliferative changes sug gestive of granulation tissue; 4, there are vascular thrombotic occlu sions; r>, lymphocyl i<; .irninitil.il on:. m.iy mii i omul some blood vonnclr.; and 6, some muitinuclcated giant colls may resoluble those seen in gran ulomas. Of course, none of these features prove an infectious etiology - t t, Z V^Vf. ft Tiia . I >w AV Ki.- .-* Fig. IS. Intraoijicplasmic filamentous and spherical oirions in a pre- cat roue lesion pl.J, . a a all early attempts to culture significant organisms in those tumors failed. In more recent times, it was demonstrated that some nuclei in the tumor cells contained structures that were somewhat suggestive of viral inclusion bodies. But electron microscopy proved conclusively that these intranuclear bodies were merely cytoplasmic invaginations. Other chemical agents in addition to the aromatic hydrocarbons have boon employed in the production of experimental brain tumors. The re* tiiiiplive L-.iii inogon:; such .is inclliy I n i I roniHire.i (MNli) and ethylnitrosourca (liNU) have tin- advantage id crop I oynb i I i I y in the rat, a larger animal than the mouse, and can be fed or injected intravenously (20, 2J). Employed in the pregnant rat, ENU passes through the placenta and produces neural tumors in the offspring (.22) . These tumors are frequent ly multiple and their locations in the nervous system are quite unpre dictable; among favorite sites are the Gasserian ganglion and the spi nal cord. Of the gliomas produced with these compounds, there seem to ho high incidences of oliqodondroqliomas and mixed gliomas. Of the nongitul tumors, both intra- and extracranial neurilemomas arc prominent. Beginning in the mid-1930s work was started with viral agents in at tempts to produce brain tumors in animals. First, the Rous sarcoma virus (RSV) was injected in chicks with some success and later, in the 1960s, the same agent was employed in hamsters, rabbits and dogs. The astrocytoma was the most common glioma produced in mammals with the RSV, but spongioblastomas and glioblastomas multiforme were not uncommon. Soon other viruses were put into use in brain tumor produc tion by an increasing number of investigators. The results of these experiments were well summarized by IKUTa and KUMANISHI (23), who re ported significant results of their own with the human adenovirus type 12 in hamsters. That the human adenovirus type 12 is oncogenic for animals is of considerable importance and has already stimulated in tensive search for other human oncogenic viruses. Most recently MUKAI and KOBAYASHI (24) described the results in newborn hamsters that re- 16 ceived : mals de' formity ependymc ported \ IN EXPLi In 1957 matogeni more imi 1 . Dlff. been es reach m functio differe lignanc the for ulated from th uted, o to dedi of the compost to prol ence of but it an epen dymal c as the tricted lymphon of manj 2. Exti spontat occurs, blastor From t; cells 1 tudinal ly far: other < tatic t followi^ tumor c< human g} seen $u( in patij shunts J The ex sumpti that e bral b and he betwee -- ; in iy ans. ve rerorgor ,nd 'quen t npte- spim to e nonnent / ,it ,i I. -ogs. th ot ducse j re> type 'r I II- Il'KAI t re ceived intracerebral inoculations of the human adenovirus. Their ani mals developed cerebral and spinal tumors of remarkable cellular uni formity that with light and electron microscopy were classified as ependymomas. H i s Mis qlmma ih.it hHkt invest wjators also have re ported when employing adenovirus type 12. IN EXPLANATION OF CERTAIN CLIOMA PROBLEMS in 1957-this writer touched on the significance of experimental gliomatoqenos i s in explaining some proMomi; with human gliomas (29). The more important of these problems are the following: 1, Differentiation and Dedifferentiation of Tumor Cells. It has long been established that the cells of the nervous system including glia reach maturity by a process of differentiation morphologically and functionally. In neoplasia, it has been assumed that a process of de differentiation accounts for the anaplastic ceils associated with ma lignancy, As part of this concept it has also been assumed that in the formation of a glioma a single differentiated ceil is somehow stim ulated to proliferation and that. I tie resulting neoplasm has its origin 1 r tain that one cell. The cellular |, I eomorpli I sin ol any glioma is attrib uted, on the basis of this theory ol the unicellular origin of tumors, to dedifferentiation. But it has already been shown in the discussion of the origin of mixed gliomas in animals that the diverse cellular composition of those tumors is a function of multicellular stimulation to proliferation. Dedifferentiation undoubtedly accounts for the pres ence of astroblasts and spongioblasts in a tumor of astrocytic origin, but it cannot conceivably account for the presence of astrocytes in an ependymoma, or oligodendroglioma, or meduLloblastoma, nor of epen dymal cells in an oligodendroglioma. The concept of dedifferentiation as the explanation for pleomorphism in human gliomas therefore has res tricted applicability. Like the extracranial neoplasms of the malignant lymphoma group that have a multicellular origin, this is also the case of many gliomas, both human and experimental. 2. Extracranial Metastasis of Gliomas. It is a well known fact that spontaneous extracranial metastasis of human gliomas practically never occurs, oven with the most malignant of the tumors such as the glio blastoma multiforme. This is also true for the experimental neoplasms. From time to time it has been reported that small numbers of tumor cells have been found in the blood aspirated from the superior longi tudinal sinus of patients with gliomas, but these tumor cells evident ly fail to survive in sufficient numbers to provide active growth in other organs of the body. When gliomas in man have been found in metas tatic foci extracranially, they have occurred almost without exception following surgery, when it was assumed that inadvertent transplants of tumor ceils occurred intravascular 1y. This writer has seen metastatic human gliomas in the lungs and liver following surgery. He has also seen such tumors lining the right pieural and the peritoneal cavities in patients who have had ventnculopleural and ventriculoperitoneal shunts for obstructive hydrocephalus due to tumors. The experimental glioma studies have helped validate the clinical as sumptions regarding extracranial metastases. First, it has been shown that experimental tumors of this class are incapable of invading cere bral blood vessel walls, unlike primary intracranial mesodermal tumors, and hence they are sealed off from effective pathways of communication between the brain and the other organs of the body. Then it was shown in 17 V numerous experiments (, 22) that such primary experimental gliomas wnen provided with an egress from the skull will grow with ease in extra cranial locations. Such egress is provided by the mechanical trans plantation of fragments of glioma with a trochar in the subcutaneous tissues of the flank in homozygous mice. Or they will grow readily in the anterior chamber of the eye in mice and other animals. In experi ments performed in my laboratory by Dr. N. KAGEYAMA some years ago, it was shown that saline suspensions of tumor homogenates when inject ed into the jugular veins of mice produced pulmonary gliomatous metastases with great ease. Likewise, such suspensions injected into the internal carotid artery resulted in intracerebral metastases. These experiments proved conclusively thal gliomas will grow extracranla 1ly if only they arc provided the necessary transport system. REFERENCES 1. KURLAND, L. T,, MYRIANTHOPOULOS, N. C., LESSELL, S.: Epidemiologic and genetic considerations of intracranial neoplasms. In: The Bi ology and Treatment of intracranial Tumors, p. 5. FIELDS, W. S. and SHARKEY, 1'. C. (uds. ) . Sprmglield, ill.: Charles C. Thomas 1962. 2. Manual of the International Classification of Diseases, Injuries, and Causes of Death: Sixth Revision, Vol. 1. Geneve: World Health Organization 1948. 3. ZIMMERMAN, II. M.; Introduction to tumors of the central nervous system. In: Pathology of the Nervous System, Vol. 2, p, 1951. MINCKLER, J. led.). Now York: McGraw-Hill 1971. 4. DORN, H. F., CUTLER, S. J.: Morbidity from Cancer in the United States. Public Health Monograph 29, U. S. Dept, of Health, Educa tion and Welfare, Public Health Service 1955. 5. HAENSZEL, W., MARCUS, S. C-, ZIMMERER, E. G.: Cancer morbidity in urban and rural Iowa. Public Health Monograph 37, U. S. Dept, of Health, Education and Welfare, Public Health Service 1956. 6. CHEN, K.-M., BRODY,-J. A., KURLAND, L. T.: Patterns of neurologic diseases on Guam. I. Epidemiologic aspects. Arch. Neurol. 19, 573 (1968). 7. COHEN, A., MODAN, B.: Some epidemiologic aspects of neoplastic diseases in Israeli immigrant population. III. Brain Tumors. Can cer 22, 1 323 (1968) . 8. ZIMMERMAN, H. M.: Brain Tumors: Their incidence and classification in man and their experimental production. Ann. N. Y. Acad. Sci. 159, 337 (1969). 9. ZIMMERMAN, H. M.: The ten most common types of brain tumor. Semi nars Roentgenol. 6, 48 (1971). 10. ZIMMERMAN, II. M.: Brain tumors. In: Methods in Cancer Research, Vol. 10, b. 105. BUSCH, II. (ed.) New York: Academic Press 1973. 11. POON, T. P., HIRANO, A., ZIMMERMAN, H. M.: Electron Microscopic Atlas of Brain Tumors. New York: Grune & Stratton 1971. 12. HIRANO, A., GHATAK, N. R., ZIMMERMAN, H. M.: The fine structure of ependymoblastoma. J. Neuropathol. Exptl. Neurol. 32, 144 (1973). 13. JELLINGER, K.: Cerebral medulloepithelioma. Acta neuropath. 22, 95 (1972) . 14. ZIMMERMAN, H. M.: The histopathology of experimental "medulloblas toma.* Acta neuropath. ,, 69 (1967). 15. ZIMMERMAN, H. M.: The nature of gliomas as revealed by animal ex perimentation. Am. J. Pathol. 3J, 1 (1955). 16. IXUTA, F., ZIMMERMAN, H. M.: Virus particles in reactive cells in duced by intracerebral implantation of dibenzanthrene. J. Neuro pathol. Exptl. Neurol. 24, 225 (1965). 18 17. POPO in r Acta 18. IKUT geni (Sup 19. CUSH 20. DRUC H. D tern Tumo Spri 21 . KOES of t Prog 22. KOES tion Spra 23. IKUT In: (ed. 24. MUKA duce Expt 25. ZIMM Slna 26. ZIMF proc 27. ZIMM Trea P. c .*5 when : a- MZ JUCt- i'i asho se .1 1'- .logic Bl- :iii s, ?alth JUS i .-,i ducaty in , of ioqic , 573 ...in cation ici. Semirh , 17 I. pic ;ure (1973) . 22, loblas11 ex! Ii neuro- 17. POPOFF, N., SUTTON, C. H,, ZIMMERMAN, H. M.: Virus-like particles in reactive cells associated with crystals of implanted carcinogen. Acta neuropath. 10, 306 (1968). 18. 1KUTA, F. , Z1MMKRMAN, II. M * : Virus particles L nducod with cnrcinugenic hydrocarbons. Proc. Rudolf Virchow med. Soc. City N. Y. (Suppl.) 26, 394 (1968). 19. CUSHING, H.: Personal communication. 20. DRUCKREY, H., IVANKOVIC, 5., TREUSSMANN, R., ZULCH, K. J., MENNEL, H. D.: Selective induction of malignant tumors of the nervous sys tem by rcsorptivc carcinogens, in: The Experimental Biology of Brain Tumors, p. 85. KIRSCH, W. M., PAOLETTI, E. G., and PAOLETTI.P. (eds.) Springfield, 111.: Charles C. Thomas 1972. 21. KOESTNER, A., SWENBERG, J. A., WECIISLER, w.: Experimental tumors of the nervous system induced by resorptive N-nitrosourea compounds. Progr. exp. Tumor Res. 1_7, 9 (1972). 22. KOESTNER, A., SWENBERG, J. A., WECHSLER, W.:Transplacental produc tion with ethylmtrosourea of neoplasms of the nervous system in Sprague-Dawley rats. Am. J. Pathol. 63, 37 (1971). 23. IKUTA, F., KUMANISHI, T.: Experimental virus-induced brain tumors. In: Progress in Neuropathology, Vol. 2. p, 253. ZIMMERMAN. H. M. (ed.). New York: Grune 4 Stratton 1973. 24. Ml'KAT, N. , KOBAYASHI, S.: Primary brain and spinal cord tumors in duct'd by hum. in .idem ,v i mu I ype 1? in hnrim t c rfi. ,7. Nciiropnthol. Exptl. Neurol. 21, 523 (1973). 25. ZIMMERMANN, H.M.: In explantion of certain glioma problems. J. M. Sinai Hosp. N. Y. 24, 1357 (1957). 26. ZIMMERMAN, H. M., ARNOLD, H.; Experimental brain tumors. I. Tumors produced with methylcholanthrene. Cancer Res. K 919 (1941). 27. ZIMMERMAN, 1). M.: Experimental brain tumors. In: The Biology and Treatment of Intracranial Tumors, p. 49. FIELDS, W. S. and SHARKEY, P. C. (eds.). Springfield, 111.: Charles C. Thomas 1962. 19