Christian and Teplitz, 1962; Ohno and Gropp, 1965) and marmosets
(Benirschke and Brownhill, 1962). Presumably this involves ex-
change of the primordial germ cells. For further discussion of
these problems see Herschler, Fechheimer and Gilmore (1966),
Kanagawa, Kawata and Ishikawa (1965), Kanagawa, Kawata, Ishikawa,
Odajima and Inoue (1966) and Makino, Muramoto and Ishikawa (1965).
Cases in which there is strong evidence for double fertili-
zation are mainly in the human and are listed in Table 6. 1In
all these cases chimerism of the blood leucocytes was associated
with a high level of chimerism in other tissues, such as the
skin and kidney. The absence of siblings is not completely satis-
factory for ruling out vascular anastomoses since it is possible
one partner died in utero after the exchange took place. Also
in domestic animals records of possible siblings are not avail-
able. Three cases are known in animals where an XX/XY chromo-
some constitution was found in either skin or kidney cells.
These are in a mouse (Russell and Woodiel, 1966), a probably
fertile male tortoiseshell cat (Malouf, et al., 1967) and a
true hermaphrodite cow (Dunn, Lein and Kenny, 1968). There are
many human cases of this type. They do not provide rigorous evi-
dence for double fertilization as they could also arise by a

sequence of mitotic errors.

The Y chromosome and testicular development

There is now a considerable number of cases of male pseudo-

hermaphrodites in goats and pigs with an XX chromosome constitution.

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