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Summary. Exposure of rabbit semen to X-irradiation did not reduce the fertilization rate until doses of >25,000 r were used, although motility was noticeably affected at 15,000 r. Evidence of a depressant effect of much lower doses of X-irradiation on the fertilizing ability of rabbit spermatozoa was obtained, however, by using a more sensitive system in which irradiated aliquots from an ejaculate were inseminated mixed together with an exactly equivalent volume of untreated semen from the same ejaculate. Under these competitive conditions the fertiliz- ing ability of spermatozoa was found not to be affected by a dose of 10,000 r, but was significantly depressed (P<0\m=.\01)by 15,000 r, as judged by an increase in the proportion of normally developing to retarded ova, recovered 50 hr after an ovulation injection of hcg. It appears that this depressant effect is exerted at the site of fertilization rather than on sperm transport. X-irradiation of spermatozoa did not increase polyspermic or delayed fertilization, neither did irradiation have any significant effect on the formation of the male pronucleus. A dose response effect was seen at the time of the first cleavage division, for at 24 hr the cleavage rate of control ova (68\m=.\5%)was similar to that of ova fertilized by spermatozoa treated with 10,000 r (68%), but only 33% of ova fertilized by spermatozoa treated with 20,000 to 25,000 r had cleaved at this time compared with 78% of their controls. At 29 and 50 hr several ova fertilized by irradiated spermatozoa remained at the pronuclear stage or at metaphase of the first cleavage division, and even at 50 hr the great majority had only reached the 2-cell stage. Thus, although it is known that cleavage can occur in the absence of the male elements, the first cleavage division does not appear to follow activation of the egg automatically and independently since it has been shown that the state of the male elements introduced at fertilization can influence the rate and ultimate success of the changes which take place at this time.

THE INFLUENCE OF X-IRRADIATION OF RABBIT SPERMATOZOA ON FERTILIZATION AND EARLY CLEAVAGE