Vle CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. 1l

found frequently in cattle (Table 1), and in sheep, goats, pigs
and cats (Tables 2,3,4). There are four possible ways in which
XX/XY individuals can arise.

The first way is by the simultaneous fertilization of an
ovum and its first polar body by two sperm carrying different
sex chromosomes, and subsequent joint development of the two cell
lines. A cytological basis for this occurrence has been found
recently by Donahue (1968) in studies on the maturation of mouse
oocytes in vitro. 1In this species the chromosomes usually scatter
within the cytoplasm of the small polar body and no metaphase II
spindle forms. However if the polar body is large the chromo-
somes line up on a spindle which cannot be distinguished from
that of the ovum. It seems likely that under these circumstances
both ovum and polar body could be fertilized.

The second way in which the XX/XY sex chromosome consti-
tution can originate is by the fusion of genetically different
male and female embryos during the cleavage stages. Such fusion
would involve prior loss of the zona pellucida from all the em-
bryos which fuse. A basis for this occurrence was first estab-
lished by Tarkowski (1961) in the mouse, who fused pairs of 8-cell
embryos. These were subsequently transferred to uterine foster
mothers and gave rise to young which exhibited retinal mosaicism,

and in some cases, true hermaphroditsm. Subsequent papers analyzed

these results in some detail (Tarkowski, 1963, 1966, a, b). Mintz

(1962; 1967) has also done independently much more extensive work