The influence of the time of spaying on the occurrence of delayed implantation has been intensively studied in the rat. Although it was originally reported that early spaying (Day 1 or 2 of pregnancy) followed by progesterone treatment did not lead to a delay in implantation, whereas spaying at a later stage (Day 4) did (Canivenc, Laffargue & Mayer, 1956;Cochrane & Meyer, 1957), further work on rats revealed that spaying before Day 4 generally resulted in delayed implantation (Psychoyos & Alloiteau, 1962;Nutting & Meyer, 1963;Mayer, 1963). Failure to observe delayed implantation more universally in the early experiments has been ascribed in part to incomplete ovariectomy. As the influence of the time of spaying on implantation has apparently not been examined in the adult mouse, the present work was undertaken. Endocrinolo-

SUMMARY The amount and distribution of prenatal mortality were studied in 126 first and second pregnancies in seventy-three rabbits, weighing 3·5–4 kg. The does were mated naturally, artificially inseminated to ensure against the failure of natural mating, and injected with 25 i.u. luteinizing hormone. The numbers of corpora lutea and implantation sites were counted at laparotomy 7–13 days post coitum (p.c.) and the number of live foetuses was recorded at autopsy 28 days p.c., or after parturition. The diameter of the uterine swellings was measured at laparotomy. In a further eight pregnancies only the litter size was recorded. Ovulation occurred in every case, but in six (4·8%) of the 126 pregnancies all the ova (109, or 8·1%) were lost before implantation. In the remaining 120 pregnancies there was a total loss of 132 ova (or 154 if corrected for polyovuly), representing 9·7% of ovulations. In 64 out of the 120 pregnancies (53·3%) there was some loss of ova before implantation. The proportion of litters showing loss and the proportion of ova lost before implantation were significantly related to the number of ovulations. After implantation, total resorption occurred in four pregnancies (3·3%) and in a further eighty-five pregnancies (70·8%) there was some loss of embryos, which altogether amounted to 18·3% of the ovulations. The proportion of litters suffering loss after implantation was significantly related to the number of implantations. It was concluded that 7% of the embryos died shortly after implantation, 66% died between days 8 and 17, and 27% were lost between days 17 and 23. Data relating to the diameter of 1728 uterine swellings measured between days 7 and 20 p.c., counts of corpora lutea, the occurrence of corpora lutea atretica and polyovuly, the position of foetuses in utero, weights of placentae associated with resorbed embryos, foetal weights, birth weights, and the incidence of stillbirths are also presented.

Single blastomeres of two-, four-or eight-cell rabbit eggs devoid of the zona pellucida, enclosed in their own zona, or injected into an evacuated host zona were transferred to the Fallopian tubes of recipient does.No single blastomeres devoid of the zona survived, while 30% 19% and 11% of those of two-, four-and eight-cell eggs enclosed in their own zona developed into normal viable young.Thirty-three out of ninety-seven single blastomeres of four-cell eggs injected into host zonae showed limited development, undergoing one or more cleavages, during the first 2 days after transfer. However, none implanted or developed into 6-day blastocysts. Failure to survive may have been due to leucocytic invasion of the zona and subsequent destruction of the blastomeres.The developmental potential of individual blastomeres is briefly discussed in relation to the production of identical offspring.

Fertilized ova have been transferred from Border Leicester to Welsh Mountain ewes, and vice versa. The oestrous cycles of the ewes were synchronized by progesterone treatment, and donor ewes were super-ovulated with PMS. 68–90 hr. after the onset of oestrus in the donors, thirty-seven ova, representing 58% of the ovulations, were recovered in vivo from thirteen ewes. Nineteen ova were transferred to eighteen recipient ewes and a total of eight pure-bred lambs were born to ewes of the opposite breed.Transfers resulted in pregnancy when oestrus in the recipients commenced from 16 hr. before to 20 hr. after that in the donors; two of the ova which developed to term were 4-cell, five were 8-cell and one was a 16-cell ovum at the time of transfer. Four out of seven and four out of eleven transfers, using blood serum and Ringer's solution respectively, resulted in the birth of lambs. Prior to transfer to the recipients seven of the ova which developed to term were exposed for 30–45 min. at room temperature, whilst one developed after exposure for 70 min.

Cell-free enzyme preparations consisting of lipoglycoprotein were obtained from acrosomes of ram, bull and rabbit spermatozoa. The ram and bull preparations showed proteolytic and hyaluronidase activities. Preparations from the three species brought about dispersal of the cumulus oophorus and corona radiata of newly ovulated rabbit eggs. In some cases the zona pellucida was also removed. Iodoacetate and a polyanionic hyaluronidase inhibitor each reduced the denuding activity of the ram preparation and to a lesser extent of the bull preparation. Heating to 100\s=deg\C also reduced the activity of the ram and bull preparations. It is concluded that proteolytic enzymes in spermatozoa contribute to the denudation of rabbit eggs and probably facilitate penetration of the zona pellucida.