THE EARLY DEVELOPMENT OJF DIDELPHYS AURITA. 91 Some Observations on the Early Development of Didelphys aurita. (Contributions to the Embryology of the Marsupialia.—V.) By I. P. Hill, D.Sc, F.R.S., Jodrell Professor of Zoology and Comparative Anatomy, University of London, University College. With Plates 6-9. INTBODUCTION. THK observations recorded in this paper are the outcome of the examination of a small amount of early material ff Didelphys aurita collected by the Percy Sladen Expe- dition in Brazil in 1918. The chief object of that expedition was the collection of embryological material of South American mammals, mid more especially of the Didelphyidse. Up to the beginning of 1916, our knowledge of the early development of the hitter rested on the well-known account given by the late Emil Selenka in 1886 of that of the Virginian opossum (Didelphys virginiana). Selenka's conclusions, as I have pointed out in my paper on the early development of Dasyurus ('10), were derived from the study of a relatively small amount of material, much of it of very doubtful value, and differ in essential respects from my own, based on the study of an abundant material of Dasyurus, : Hnd as we should not expect a priori to find fundamental •differences in the early development of two animals so closely related as Didelphys and Dasyurus undoubtedly are, the need for a l'e-investigation of the early development of the former ..was sufficiently obvious. "92 ' • • J. P. HILL. In the interval that need has been met by the publication in March of last year of an extremely valuable account of the early development of Didelphys virginiana, by C. G. Hartman, of the University of Texas ('16). For another reason, we were anxious to secure develop- mental material of the Didelphyidee. It has been suggested that the Australian marsupials have been derived from an ancestral Didelphyd stock, and if that be so, it is of first importance that we should possess a knowledge of the development of those genera of the Didelphyidse which on anatomical grounds have been regarded by various authorities (Winge ('93), Bensley ('03), Bresslau ('12) and others) as lying nearest the base of the Didelphyd series. The genera in question, Marmosa and Peramys, are small, rat-like forms, remarkable for the entire absence of the pouch so charac- teristic of most other members of the order. In view of the researches of Bresslau ('12), which, demonstrate that the pouch of the marsupial has nothing whatever to do with the pouch or incubatorium of the monotrematous Echidna, it may well be that the non-existence of a pouch in these genera is a further evidence of their primitive character. We were accordingly most anxious to obtain embryological material of these pouchless forms. Developmental material of non-domesticated mammals just in the precise stages desired by the investigator is at no time very ensy to obtain, and that inherent difficulty was enhanced in our own case by the absence of exact data as to the breeding seasons of the South American forms, and of records of their frequency and local distribution. As it turned out, however, we were fortunate in that we found shortly after our arrival in Rio de Janeiro that Didel- phys aurita (the Gamba of the Brazilians), a species widely distributed throughout Brazil and relatively common in certain region?, especially near habitations, was breeding. We collected altogether some forty-six specimens of this fox'm, and of these, eight proved to be pregnant females, nineteen were females with pouch-young, three were non- THE EABr,Y DEVELOPMENT OF DIDELPHYS AUBITA. 93 gravid females, smd sixteen were males. As concerns the poucliless forms, although we succeeded in trapping a number of specimens of Mavmosa ci,nerea, and in securing, thanks to the kindness of Mons. A. Touchon, A.M.T.C.E., a few specimens of Peramys americana and of P. scalops,1 from the Theresopolis region (Serra dos Orgaos),we unfortu- nately obtained no developmental material. These forms, though not perhaps rare, are shy, retiring creatures, some- what difficult to trap, and to secure them in numbers, one would require to live in the regions in which they occur for a considerable time.2 The material dealt with in this paper and listed below was derived from six females of D. aurita, and comprises eggs from the unsegmented condition up to the 16-celled stage, and blastocysts of about 1 mm. in diameter. It is thus of a somewhat fragmentary nature, but is, I think, worthy of description, inasmuch as it is derived from a different, though closely allied, species to that from which Hartman obtained his material. Moreover, although my observations, so far as they go, are in the main confirmatory of the latter's results, I believe I am able to supplement his work in regard to certain not unimportant details, e. g. the mode of origin of the characteristic cross-shaped, 4-celled stage. I have to express my grateful thanks to the Council and Government Grant Committee of the .Royal [Society and to the Percy Sladen Trustees for generous grants in aid of the Expedition. My friend and colleague, Major G. S. Samson, M.C., R.F.C., B.Sc, rendered me loyal co-operation through- out, and 1 much regret that existing circumstances have prevented him from collaborating with me in the preparation of the present paper, which forms the first of a series dealing ' I am indebted to Mr. Oldfield Thomas for the specific determina- tions. 3 For an account of the habits, etc., of the three-striped opossum (P. americana [tristriata]), vide Goeldi, E, A., "Critical Glean- ings on the Didelphyidre of the Serra dos Orgilos, Brazil," ' P. Z. S.,' 1894. 94 ' J. P. HILL. with the material collected by the Expedition. I desire also to acknowledge the generous help afforded us by scientific men in Brazil. With the permission of the Minister of Agriculture, we were enabled to utilise the well-equipped laboratory of the Jardim Botanico as our headquarters, and to Dr. J. C. Willis, late Director of the Gardens at Rio, we are under a deep debt of gratitude for unstinted kindness and help during our stay. We nre also indebted for in- valuable assistance to Dr. A. de M. Ribiero, of the Inspectoria da Pesca, and his staff; to the staffs of the Jardim Botanico and the Museu Nacional; and to Dr. Oswaldo Cruz, Director of the Institute Oswaldo Cruz, and to Dr. A. Lutz, of the same Institute.1 Lnstly, we desire to thank G. Chalmers, Esq., Superintendent of the St. John del Rey Mining Co., and liis staff, for many facilities and much kindness during our stay lit Morro Velhyo. LITERATURE. At the time the material described in this paper was collected, Selenka's account of the early development of D. virginiana was, as already mentioned, the only one avail- able. That account has already been critically dealt with in my paper on Dasyurus ('10) and also by Hartman in his recent paper, and to these critiques the reader is referred. Shortly after the return of the Expedition to England, I learned from Prof. J. T. Patterson, of the University of Texas, that one of his research workers, Mr. C. G. Hart- man, had been collecting developmental material of the American opossum and had obtained a very complete series of stages. I therefore decided to await the publication of Mr. Hartman's results before describing my own fragmentary material. Prior to the issue of Mr. Hartman's paper, how- ever, I had made out that the Didelphys ovum is considerably smaller than that of Dasyurus, that the deutoplasm is elimi- nated in a different fashion, that there is a cross-shaped 4-celled 1 Since the above was written, we have seen, with deep regret, the announcement of the death of Dr. Cruz. THE EARLY DEVELOPMENT OF MDELPHYS AURITA. 95 stage, and that in the 16-celled stage there is not that clear evidence of the differentiation of the blastomeres into two groups, respectively formative and non-formative, which is so striking in Dasyurus. Mr. Hartman's paper ('16) nppeared in March of last year, and proves to be a very complete and excellent piece of work.. In the main, his results are confirmatory of my own for Dasyurus and entirely justify the criticisms of Selenka's work expressed in my paper. Furthermore, his results, taken in conjunction with my own, now justify us in concluding that the early development of the Marsupialia is effected throughout according to a common plan, such differences as exist being differences in detail and not in principle. • Mr. Hartinan shows that the ova of D. virginiana, as was apparent from Selenka's description, are considerably smaller than those of Dasyurus; "its egg measures on the average about one-half the diameter, that is, one-eighth the volume of the egg of Dasyurus." They exhibit no obvious polarity inasmuch as there is no polar concentration of deuto- plasmic material and in correlation therewith elimination of surplus yolk occurs, not by the separation of a yolk-body at one pole as in Dasyurus, but by the elimination of numerous small masses ("yolk-spheres") from the entire periphery of the blastomeres in the 1- and 2-celled stages, in a manner comparable with that described by Van der Stricht ('09) for the ovum of Vesperugo. The first cleavage plane divides the ovum into two approxi- mately equal cells, but, whilst there nre no recognisable quantitative differences between the blastomeres, Hartman indicates that there is possibly a qualitative difference, inas- much as he observed in one case that one of the two blasto- meres eliminated its yolk-material prior to the other.