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There has been considerable interest in recent months in the possibility of using fertilization in vitro and embryo transfer techniques to alleviate certain forms of human infertility. These techniques have been used successfully in the rabbit (Chang, 1959; Fraser & Dandekar, 1973), mouse (Whittingham, 1968; Hoppe & Pitts, 1973) and rat (Toyoda & Chang, 1974). In general, relatively few embryos have been transferred and little reference to the offspring has been made other than to note phenotypic normality. Fertilization in vitro has also been described for human eggs (Edwards, Steptoe & Purdy, 1970) and more than twelve such eggs have subsequently been transplanted (Robertson, 1974), although no pregnancies have yet been reported (de Kretzer & co-authors, 1973). The reasons for failure are unknown, but the possibility exists that fertilization in vitro may increase the chances for union of defective or abnormal gametes. Under conditions of natural mating, relatively few spermatozoa from the ejaculate reach the site of fertilization (Piko, 1969), and there are opportunities for sperm selection (Krzanowska, 1974), but with fertilization in vitro, all sper¬ matozoa presumably have an equal opportunity to fertilize an egg. It is, therefore, desirable that experimental data comparing the incidence of abnormali¬ ties in the products of eggs fertilized in vivo and in vitro be determined before the techniques of fertilization in vitro and embryo transfer are applied to man. Toyoda & Chang (1974) examined the offspring derived from rat eggs fertilized in vitro. These authors reported that eleven of twenty-two rats showed signs of microphthalmia while the defect was not seen in a group of control animals of similar size. Development of microphthalmia in the rat apparently involves both genetic and physiological factors (Hain, 1937). In a study of fourteen rabbits derived from eggs fertilized in vitro, Fraser & Dandekar (1973) described two animals with splayleg. None was found in a similar number of control rabbits derived from naturally fertilized eggs cultured in vitro and trans¬ ferred to pseudopregnant recipients. There is no information on the natural incidence of splayleg in rabbits, but the defect is known to be inherited and dependent on a recessive autosomal gene (Innes & O'Steen, 1957). We have studied the chromosomes in ten of the twelve phenotypically normal experimental rabbits mentioned above (Fraser & Dandekar, 1973). As controls, two age-matched naturally bred New Zealand rabbits were examined. Blood,

CHROMOSOMAL ANALYSIS OF RABBITS DERIVED FROM EGGS FERTILIZED IN VITRO