Helen Spurway, a geneticist specializing in the reproductive biology of the guppy, "Lebistes reticulatus", claimed, in 1955, that parthenogenesis, which occurs in the guppy in nature, may also occur (though very rarely) in the human species, leading to so-called "virgin births". This created some sensation among her colleagues and the lay public alike. Sometimes an embryo may begin to divide without fertilisation but it cannot fully develop on its own, so while it may create some skin and nerve cells, it cannot create others (such as skeletal muscle) and becomes a type of benign tumor called an ovarian teratoma. Spontaneous ovarian activation is not rare and has been known about since the 19th century. Some teratomas can even become primitive fetuses (fetiform teratoma) with imperfect heads, limbs and other structures but these are non-viable. However, in 1995 there was a reported case of partial-parthenogenesis; a boy was found to have some of his cells (such as white blood cells) to be lacking in any genetic content from his father. Scientists believe that in the boy's case, an unfertilised egg began to self-divide but then had some (but not all) of its cells fertilised by a sperm cell; this must have happened early in development, as self-activated eggs quickly lose their ability to be fertilised. The unfertilised cells eventually duplicated their DNA, boosting their chromosomes to 46. When the unfertilised cells hit a developmental block, the fertilised cells took over and developed that tissue. The boy had asymmetrical facial features and learning difficulties but was otherwise healthy. This would make him a parthenogenetic chimera (a child with two cell lineages in his body). While over a dozen similar cases have been reported since then (usually discovered after the patient demonstrated clinical abnormalities), there have been no scientifically confirmed reports of a non-chimeric, clinically healthy human parthenote (i.e. produced from a single, parthenogenetic-activated oocyte).