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THERE have been several attempts to activate mammalian eggs to parthenogenesis. Pincus et al.1–8first showed that rabbit eggs can be activated in vitro or in situ with various physical and chemical agents and that some of these eggs can begin to develop. Four living young were born which were considered to have been of parthenogenetic origin3,4,7,8. Thibault9–11and Chang12,13, working with eggs of the same species, confirmed earlier suggestions that cold shock is an effective parthenogenetic agent and that eggs so activated in vitro or in vivo can develop to blastocysts. But although some parthenogenetic blastocysts were morphologically normal and diploid13, development beyond implantation was never observed11,13.

Histological examination of the reproductive systems of fourteen newborn chimaerae developed from fused eggs revealed eleven sexually normal animals and three true hermaphrodites. The genital apparatus of the hermaphrodites is described, the structure of the gonads being given special attention. Of the three hermaphrodites, one had one ovary with female sex ducts and an ovotestis with complete male ducts and incomplete female ones. The second had two ovotestes and a genital apparatus of nearly typically male type ; the remnants of Müllerian ducts being retained only caudally. The third had one ovary with female sex ducts and one ovotestis with male sex ducts. The occurrence of true hermaphrodites strongly suggests that spontaneous true hermphroditism also develops from sex-chromosome mosaicism. The available literature on spontaneous hermaphroditism in rodents is critically reviewed. It is postulated that the frequencies of the four types of true hermaphroditism described (ovary-testis, ovary-ovotestis, testis-ovotestis, ovotestis-ovotestis) in mice and other rodents do not adequately reflect their actual incidence in nature in that the incidence of the lateral type of hermaphroditism is exaggerated. It is suggested that if the testicular tissue does, in fact, elaborate a morphogenetic stimulus influencing the differentiation of XX gonadal tissue, such a stimulus must be rather weak and act only near the site of its elaboration. The genetic sex of germ cells in chimaeric hermaphrodites remains unknown. It is postulated that the presence in the hermaphrodite gonads of oöcytes and male germ cells reflects only the secondary differentiation of germ cells, which need not reflect their genetic sex. While the ovarian tissue of the ovotestes examined contains only oöcytes, in the testicular tissue both oöcytes and spermatogonia are present. Initiation of meiosis of the female type in the testicular territory is attributed to the action of a meiotic stimulus elaborated by the ovarian tissue and diffusing through the testicular part.

Distribution of pigment-forming cells has been analysed in sixteen individuals (six embryos, nine newborn young, one adult animal) developed from fused eggs differing in genotype for pigmentation (pink eye dilution versus agouti). The following structures were examined: the outer layer of the retina (all individuals), choroid and Harderian gland and skin from various regions of the body (postnatal individuals). In the six embryos, pigment was revealed in their retinae but since the development of pigmentation was still in progress at that age the genotype of non-pigmented cells could not be identified. In seven out of the nine newborn young the outer layer of the retina was a mosaic with pigmented and non-pigmented cells intermingled. In the two other animals the outer layer of the retina was homogenous, being of’pink’ type in one animal and of ‘agouti’ type in the other. In the adult animal the outer layer of the retina of one eye seems to be composed of two types of cells. The difference of pigmentation between them is quantitative only, the colour and shape of the granules being the same and typical for the agouti (hybrid) component. To postulate the chimaeric constitution it would be necessary to assume, therefore, that the cells derived from the LAB Grey component (pink-eyed) have been in some way induced by the neighbouring hybrid cells to synthesize pigment typical of the latter. In all newborn animals, except the one with non-pigmented eyes, melanocytes from the agouti component were detected in the choiroid, Harderian gland and the samples of skin examined. However, the presence of melanocytes of the other genotype could be neither proved nor rejected. On the basis of the composition of the retinae and the distribution of pigmented melanocytes in the skin it is inferred that the majority of animals developed from fused eggs are real chimaeras, at least as far as the neural derivatives are concerned. Since pigmentation does not reveal the genetic constitution of structures of different developmental history and derivation, the chimaeric character of animals with homogenous retinae cannot be excluded.

Mouse 2- and 8-cell and rat 2-, 4-, and 8-cell eggs develop into blastocysts when reciprocally transferred to the oviduct of the other species. Some rat eggs, after attaining the blastocyst stage in the mouse oviduct, do not shed the zona pellucida at the normal time and continue to increase in volume beyond the normal. This is accompanied by spreading of cells of the inner mass beneath the trophoblast, leaving the blastocyst without any compact embryonic structure. Other eggs develop into normal blastocysts with an inner cell mass present. Mouse eggs in the rat give rise to normally built blastocysts. Both kinds of rat blastocysts—hollow and normal—and mouse blastocysts evoke a typical decidual reaction in the uterus of the foreign species. Normal crypts are formed with the blastocysts attached to the epithelium. These processes begin well before any changes in the uterine epithelium become visible. The blastocysts of neither species are able to secure a normal contact with the mucosa. Degeneration of the epithelium under the influence of the trophoblast is delayed, irregular, and only small areas are involved. In the mouse, the eventual denudation of the mucosa around the blastocyst is thought to be largely an effect of spontaneous degeneration of the epithelium due to oedomatous changes in the underlying mucosa. The survival of mouse and rat blastocysts after implantation is very limited, the upper time limit being, in the mouse, the 7th day and, in the rat, the 8th day. Those rat blastocysts which have succeeded in establishing a contact with the mucosa undergo some differentiation, i.e. form small egg cylinders. This is accompanied by growth which in some cases is, however, very slight. Mouse blastocysts in the most successful cases exhibit immense increase in size but neither differentiation nor growth have been observed in their inner cell masses. The kidney surface of both species provides a favourable environment for the development of rat and mouse blastocysts reciprocally transferred. The trophoblast gives rise to numerous giant cells. The cells presumably of ‘inner mass’ origin are also visible, but organized embryonic development has not been observed. A number of rat eggs which developed into blastocysts in the mouse were re-transferred to rat recipients. The subsequent mortality rate was very high. Three purely trophoblastic vesicles were encountered and it is believed that they have developed from hollow blastocysts. These findings are discussed in the light of embryology and reproductive physiology.

On the 1st day after mating, the oviducts of the mouse are infected with bacteria. Infection is eliminated within 24 to 36 hr, presumably by the leucocytes appearing at this time in the genital tract. Two groups of strains, one producing a greenish discolouration of the blood and the other β-haemolytic, were isolated from the oviducts and were identified as being most probably varieties of enterococci. In the bacterial flora of vaginae, non-haemolytic streptococci, closely related to those isolated from the oviducts, predominated.

The purpose of the present study was to trace the fate of primordial germ cells in mouse chimeras of XX/XY constitution. In this type of research one hopes to obtain knowledge of the role of intrinsic genetic factors and of environmental factors (the environment provided by the gonads) in initiating and directing the course of gametogenesis in mammals.

A considerable amount of information bearing upon mouse chimeras obtained experimentally by fusion of cleaving eggs has already been accumulated (Tarkowski, 1961, 1963, 1964a, b, 1965; Mintz, 1962a, b, 1964a, b, 1965a, b, 1967). Direct and indisputable proofs of the chimerism of the individuals produced were obtained by establishing that two types of pigment cells were present in the outer layer of the retina (Tarkowski, 1963,1964a) and in the coat (Mintz, 1965a, b, 1967). The cell composition of other organs and tissues has not as yet been investigated. True hermaphroditism in some of the animals obtained was also accepted as an indirect proof of chimerism (Tarkowski, 1961, 1963, 1964b; Mintz, 1965b) but the postulate of these individuals being six-chromosome chimeras has not as yet been directly confirmed by karyological research.

Organization of a mammalian egg at the beginning of development has been the subject of several experimental as well as descriptive cytological and cytochemical investigations.

When blastocysts lacking zonae pellucidae were transferred to the reproductive tract of female mice, several implantations were subsequently found outside the tract, in the peritoneal cavity. These were presumably derived from blastocysts which had escaped from the reproductive tract. Eggs with and without zonae pellucidae were then injected directly into the peritoneal cavity : the incidence of implantation was low, and did not lend any support to the hypothesis that absence of the zona pellucida might facilitate extra-uterine implantation. In all, eight intraperitoneal implantations were found. Two contained recognizable embryonic structures, of which one closely resembled a normal 8- to 8½-day embryo.