on the fusion of mouse embryos and proposes they be called
allophenic mice. While the experiments of these two investi-
gators may be helpful in the study of differentiation, there is
no evidence that fusion of independent embryos at the cleavage
stages is a mechanism for the production of intersexes under
natural conditions.

The third way in which an XX/XY sex chromosome
constitution can originate is by a sequence of mitotic errors in
an XY zygote. The phenomenon involves loss of the Y chromosome
by anaphase lag, non-dysjunction of the X chromosome in the new
X0 cell line, and loss of the 00 and XO cell lines. The proba-
bility of this sequence of events is of course extremely low.

The fourth way in which an XX/XY sex chromosome
constitution can arise is by the fusion of the chorions and the
development of vascular anastomoses between heterosexual sib-
lings. The latter phenomenon occurs frequently in cattle(Lillie,
1917) and in marmosets (Hill, 1926; Wislocki, 1932), and allows
the two way exchange of cells between the foetuses. That an ex-
change of cells does occur was indicated by the work of Owen
(1945) who showed that cattle twins had identical blood groups.
Benirschke, Anderson and Brownhill (1962) and Benirschke and
Brownhill (1962) showed the presence of XX/XY chimerism in
bone marrow in the twins of several species of marmoset, and
Fechheimer, Herschler and Gilmore (1963) demonstrated
XX/XY chimerism in the blood leucocytes of several cases of

breemartins and their male partners. There is now evidence

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