The influence of maternal and genetic factors on the size of lambs at birth and on their gestation length has been studied from the results of two experiments involving the transfer of fertilised eggs from one breed of sheep to another. In the first experiment, eggs were transferred reciprocally between old ewes of the large Lincoln breed and of the small Welsh Mountain breed. Results are based on 20 transfer lambs and 14 normally born singles. In the second experiment, 35 pure Lincoln and 28 pure Welsh lambs were born as a result of transferring eggs into 18-month-old nulliparous Scottish Blackface sheep.The breed type of the lamb was found to be the most important factor influencing weight and cannon length at birth, although maternal-foetal interactions were present. As the maternal environment became poorer, genotypic differences remained distinct but decreased in magnitude. The maternal environment provided by 4th parity Lincoln ewes, if judged by lamb's size at birth, was better than that provided by 4th parity Welsh ewes, and this in turn was better than that provided by 1st parity Blackfaces. Lincoln and Welsh lambs in the same maternal environment differed in average birth weight by 5·7, 3·8 and 3·0 1b. respectively in the Lincoln, Welsh and 1st parity Blackface maternal environments. Lincoln and Welsh lambs, each in their own normal maternal environment, differed by 6·5 lb. The maternal influence on cannon length was similar but small. Gestation length of Lincoln lambs in Welsh recipients was almost 4 days shorter than that of Welsh lambs in Lincoln recipients. In the Blackface maternal environment, Lincoln and Welsh lambs differed little in gestation length but males were, on average, carried about a day longer than were females.At the individual level, weight of (Blackface) recipient ewe had a somewhat greater effect on lamb's size than had weight of (Lincoln or Welsh) donor ewe. The magnitudes of these ‘genetic’ and ‘maternal’ effects were comparable with those found at the level of group means; even so, donor's and recipient's weight together accounted for only a small part of the individual variation in lamb's size about the group means.It was concluded that a ewe was able to respond to progressively greater demands made by a lamb or lambs of progressively larger (combined) potential size—but in accordance with the law of diminishing returns—and that the upper limit of maternal accommodation must be considerable. By contrast, the upper limit of mean genetic size at birth appears to be not far above the mean size of single lambs born to their natural mothers in good condition. A model has been constructed from which mean birth weight of lambs can be predicted.

Summary. A technique is described for the surgical recovery of ova from the sow's uterus. With this technique 118 ova were recovered from eight of a total of nine sows operated on 3 to 5 days after the onset of heat. A total of eighty-six ova were transferred to the uteri of six sows of which three farrowed. These three sows had received eighteen, fourteen and ten ova and gave birth to litters of twelve, thirteen and six pigs, respectively. The results are discussed in relation to factors affecting the fertility of inseminated pigs.

There was no evidence of any effect of the time of storage in vitro (up to 5J hr) on the success of transfers among ova transferred on the day of recovery.

Forty-eight sows were inseminated into the uterus with semen (sperm fraction) diluted to 20 ml. with an egg-yolk glucose phosphate (YPG) diluent to contain 10·0 × 109, 1·0 × 109 and 0·1 × 109 spermatozoa. In 24 sows (120 ml. group) insemination of the 20 ml. diluted semen was followed by the intra-uterine insemination of 100 ml. YPG diluent, alone. The success of insemination was judged at autopsy from ova recovered 2-4 days after insemination (Experiment A) or from counts of foetuses present 25 days after insemination (Experiments B and C). In Experiment A the numbers of ‘cleaved’ ova were recorded before fixation: the numbers of ova judged to be ‘fertilised’ were counted in preparations of fixed and stained ova. For 24 sows inseminated with 20 ml. semen alone at the three dosage levels (10·0 × 109, 1·0 × 109 and 0·1 × 109 spermatozoa) the percentages of ‘cleaved’ and ‘fertilised’ ova were 78·5 and 80·9; 83·5 and 81·2; 24·6 and 19·8. For sows inseminated with the same sperm numbers in 20 ml. semen followed by 100 ml. diluent, the percentages were 41·5 and 43·9; 44·2 and 36·2; 24·6 and 6·7. Comparison of numbers of sows with some ova cleaved (or ‘fertilised’) and sows with none cleaved (or ‘fertilised’) showed no significant differences in sow fertility between treatments.Comparison of numbers of sows with all ova cleaved (or ‘fertilised’) and sows with none cleaved (or ‘fertilised’) shows that inseminations with the 20 ml. volume, alone, were of significantly higher fertility than those inseminated with 120 ml.Estimates of fertility based on the examination of unfixed ova differed in 9 sows from estimates based on the examination of fixed and stained ova. In one sow some uncleaved ova were shown to be pronucleate after fixation and staining. Morphologically abnormal ova were identified in 6 sows; most of these abnormal ova showed multinucleate blastomeres. These abnormal ova were found only in sows inseminated with not more than 1·0 × 109 spermatozoa.Counts of the numbers of spermatozoa on fixed ova showed that these tended to decline as the number of spermatozoa inseminated declined. Comparing sows inseminated with the same number of spermatozoa there tended to be fewer spermatozoa on ova from sows inseminated with 120 ml. than on ova from sows inseminated with 20 ml.Of 13 sows in Experiment B inseminated with 20 ml. semen containing 10·0 × 109 spermatozoa, 9 had foetuses present at autopsy 25 days later (11·6 foetuses per sow).Of 17 sows in Experiment C inseminated with 20 ml. semen containing 1·0 × 109 spermatozoa, 7 had foetuses present at autopsy 25 days later (7·1 foetuses per sow).The difference in pregnancy rate for the two groups is not statistically significant.Differences in litter size between the two groups of sows are explainable by differences in ovulation rate between the two groups (group B, 18·2 corpora lutea per sow, group C, 13·4 corpora lutea per sow).

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