Bacteria can be found in the reproductive tracts of both sexes; some of them are highly spermicidal and may be responsible for infertility (Matthews & Buxton, 1951; Teague, Boyarsky & Glenn, 1971). It has been reported that several viruses can infect and damage implanted mammalian embryos (Sever & London, 1969).
In rabbits, the blastocyst is susceptible to viral and bacterial infections only when implantation has begun (Zimmermann, Gottschewski, Flamm & Kunz, 1963). By contrast mengo-encephalitis virus is capable of infecting, and blocking the further development of, two-cell mouse eggs and morulae in vitro (Gwatkin, 1963, 1967). This communication describes the bacterium-like particles in four rat blastocysts during delayed implantation.
Five 30-day-old female rats of Holtzman strain were induced to ovulate with PMSG (Gonadogen, Upjohn Co.) followed by hcg (International Hormones, Inc.), and were mated with fertile males (Wu & Meyer, 1966). Implantation of blastocysts was prevented by ovariectomy of the
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