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(1) A large series of data on the duration of gestation in the goat has been analysed and the constants calculated.(2) The frequency curve is unimodal and symmetrical, suggesting a single factor or a variety of factors acting simultaneously as the cause for birth.(3) Slight breed differences exist in the duration of gestation in the goat as in other species examined.(4) There is a continuous variation in the duration of gestation with the time of year at which conception occurs. Spring conceptions give shorter gestations than autumn conceptions.(5) There is a distinct difference in the duration of gestation for young and older dams. This is related to the age of the dams and not to the order of the births. Gestation is shorter in the young animal than in the older.(6) In the goat the size of litter has little or no effect on the duration of gestation.(7) Constants for duration of gestation in other species have been obtained. Variability increases with the duration of oestrus and about half the variability in any species with a long oestrous period may be ascribed to this cause.(8) The factors involved in fixing the duration of gestation in the species examined are evidently the same in all cases, and probably culminate in a single agency responsible for terminating gestation.

The duration of gestation in dairy cows is of importance to the dairy husbandman from a management standpoint, for it is desirable that dairy cows be in a no~l-lactating state during the last 6 to 8 wk. of each pregnancy. Thus, it is possible to prepare the cow physically for parturit ion and her next lactation. Dairy cattle breed associations also require that a living calf be produced within a maximum time limit after the completion of certain types of official production records. The purpose of this requirement is to assist in verifying the duration of pregnancy during the Official testing period. Recently the Brown Swiss Cattle Breeders' Association has extended the calving requirements following the completion of an official record to conform with the abnormally long gestation period found to exist in the Brown Swiss breed. Likewise, the Holstein-Friesian Association has reduced the gestation length used in the verification of applications for registration. This decision was based on the preliminary results of this study and data from similar investigations. In addition to these very practical applications, the possibility of various environmental and genetic factors influencing the duration of gestation is intriguing and may lead to fundamentally important discoveries. The commonly accepted opinion at the beginning of the nineteenth century was that normal gestations for dairy cattle averaged 270 days (33). Systematic investigations soon revealed this figure to be less than that normally observed. In 1817, Tessier (8) is reported to have found that 575 gestations averaged 282.2 days. Then, in 1840, the Earl of Spencer (33) is reported to have made ade tailed study of Durham gestatimm. His results showed the average length of 764 gestations to be 283.3 days. The range for calves born alive, reported by Spencer, was from 220 to 313 days. However, no calves born earlier than 242 days lived. Over half a century ago, Fleming-(18) reported the average gestation of cattle to be 283 days. Of the 1,062 observations included in his summary, 15 gestations were less than 241 days and 32 greater than 301 days.

ウマの在胎日数についての研究は古くから行なわれているが報告者によりその値が異なっている.適確な分娩予定日の決定はウマの管理省力化よりの要求が高い.今回著者らは,同一場所で長期間繋養された群について在胎日数変動要因の追求を行なった.材料は1948年2月から1972年7月までに農林省十勝種畜牧場において3回以上分娩を繰返した繁殖雌馬192頭,延べ954回の分娩例の在胎日数である.統計学的分析を行ない,次の結果を得た.1. 在胎日数は最短286日,最長371日であった.2. 在胎日数について個体,分娩月,品種および胎児の性などの要因別に分散の検定を行なった結果,有意性が認められた要因は個体(P≤0.01),分娩月(P≤0.05),および品種内ではアラブ種(P≤0.05)であり,反対にブルトン種は非常に変動が少なかった.3. 在胎日数の棄却限界と信頼限界(信頼度99%)を分娩月,品種および胎児の性別などについて求めた.棄却限界と信頼限界はそれぞれ307.9日から363.5日までと326.4日から343.5日までの範囲内にあった.4. 段階式分類により在胎日数について分散分析を行ない,遺伝力の推定を行なった.その結果同一父の半きょうだい,同一母の半きょうだい,そして全きょうだいなどの相関からの遺伝力推定値はそれぞれ24%,97%および60%であった.

長崎大学学芸学部自然科学研究報告. vol.15, p.17-40; 1964

Summary. Results of egg transfer and of reciprocal crossing among breeds of sheep with a range of 1 week in mean gestation period indicate that the genotype of the fetus is the major determinant of the duration of gestation, over a range of litter sizes from 1 to 5 and a wide range of combinations of size of donor and recipient breeds. It is concluded that the fetus accounts for at least two-thirds of the genetic variation in gestation period in this species. Evidence for the role of the fetus in determining the duration of gestation comes from two independent sources, namely endocrine studies of the fetal pituitary-adrenal axis (Liggins, Kennedy & Holm, 1967; Liggins, 1969), and genetic studies, with the most critical evidence provided by interbreed egg transfer (Dickinson, Hancock, Hovell, Taylor & Wiener, 1962; Moore, 1968). We present here the results of interbreed egg transfer and crossbreeding experiments involving breeds with a wider range of gestation periods than have been used in earlier experiments. They provide striking evidence of the role of the fetal genotype in determining the duration of gestation in this species. Descriptions of the animals involved in the egg transfer experiment and of the procedures used for synchronizing oestrus in donors and recipients have been reported by Bradford, Quirke & Hart (1971). The time of onset of oestrus was recorded twice daily, and the actual time of parturition was recorded to the nearest hour, except for ewes lambing unattended during the night for which parturition was listed as occurring at 07.00 hours. The duration of gestation was calculated to the nearest half day. Where donor and recipient ewes were not exactly synchronized, the time of onset of oestrus in the donor was used as the basis for computing the gestation period; all but seven of the fifty-three recipient ewes for which gestation periods are reported were synchronized within 12 hr of the donor. In addition to the pregnancies following egg transfer, data were obtained on seventy-seven control pregnancies, mostly of donor ewes, following intra-breed mating. The same rams, in most cases one or two per breed, were used at the oestrus from which eggs were collected and on the control group. Extensive data were also available on gestation periods from the parental flocks. Lambs in the experiment were born during a 3-month period, March to

Summary. The negative correlation between the number of foetuses and the length of gestation characteristic of most multiparous species is not found in the pig. This observation, reported in previous studies, was confirmed using 1450 records from the Duroc and Hampshire breeds. The gestation period was also unrelated to either the number oflate foetal deaths or to the weight and fatness of the sow but prolonged gestation at first parturition reduced the probability of a second litter. Biggers, Curnow, Finn & McLaren (1963) have pointed out that the duration of gestation varies inversely with the number of foetuses in mice, guinea-pigs, cattle, sheep, goats, rabbits and humans. None of the seven studies on ges¬ tation period in pigs, cited by these authors, showed any significant relation¬ ship between the number born and the length of gestation. The purpose of this brief report is to substantiate the existing evidence in the pig and to examine other sources of variation among 1450 gestation periods observed in 693 purebred Duroc and Hampshire sows. The animals were assembled in 1959 to investigate the genetic effects of irradiation. Willham & Cox (1962) have given details on the source of the original animals and the mating plan used in the herd. The length of gestation was recorded as the difference, in days, between the date of mating and the date of farrowing. The distribution of the duration of gestation is given in Table 1. The 1450 pregnancies comprise 750 in Hampshire sows and 700 in Duroc sows. The average gestation period was 113-24 days in the former, and half a day longer in the latter, the difference being highly significant. The variances and covariance of the litter size at birth and the length of gestation were obtained by pooling values calculated within groups of sows of like breed and age that were served by the same boar in the same season. The linear regression coefficient for the regression of the gestation period in days on the number of pigs born in the litter was —0-016±0-020. The correlation was —0-03. Thus, in these data, as in previous studies, no significant relation¬ ship was found between litter size and duration of gestation. Other factors were examined in a similar manner for their possible influence on the length of gestation. The incidence of late foetal death, measured by the