Vle CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. lI

that the exchanges of cells between foetuses occur in several
other species, including sheep, pigs, goats and human beings.
The differential diagnosis of these four causes of XX/XY
chimerism cannot be done with certainty. There is no way of
distinguishing individuals produced by double fertilization,
from allophenes and by mitotic errors, as all are expected to
give rise to individuals in which the majority of tissues are
mosaic. Individuals in which cellular exchange occurred by va-
scular anastomosis, however, may only have very few of their
tissues chimeric, depending on the cells in the blood stream
which had an opportunity to exchange. Thus a thorough cyto-
logical examination of all cases of intersexuality should include
the study of the chromosomes in several tissues-blood leucocytes,
kidney, lung, skin, gonads, etc. If an XX/XY chromosome con-
stitution is found only in cultures of the blood leucocytes it
is assumed the chimerism arose by vascular exchange of cells.
If the XX/XY chromosome constitution is found in cultures of
several types of cells it is assumed that the chimerism arose
by the simultaneous fertilization of an ovum and its first polar
body. These assumptions are based on the belief that the pro-
genitors of skin and kidney cells are not mobile in early de-
velopment and are not likely to pass from one circulation to
another. There is now good evidence that germ cells may also

be exchanged between foetuses in cattle (Ohno, Trujillo, Stenius,

855