A quantitative study of reproduction in some species of Ceriodaphnia (Crustacea: Cladocera)

The world distribution of species of the genus Ceriodaphnia has been compiled from the literature and is summarised. The nomenclature of C. cornuta is discussed.The present study is based on collections of C. pulchella, C. reticulata, C. megalops and C. laticaudata from the Long Water and the Wick Pond of Hampton Court, Middlesex during the period April 1963 - July 1964. The seasonal variations in the size of parthenogenetic females and of their reproductive capacity were investigated and are recorded. The relationships of egg numbers and egg volumes to body lengths and to the temperature and chlorophyll content of the water are analysed and discussed.The occurrence of sterile eggs in Ceriodaphnia pulchella and C. reticulata during 1963 is recorded. A few details of body size, egg number and egg size of an unidentified species from Malta are given and compared with those of the British species. The results of these recordings and analyses are discussed in relation to the principle of the ecological niche and the idea that size difference is a prerequisiteof cohabitation of species of the same genus. The results are also discussed in the light of those of other authors' work on other cladoceran species.<p

A randomized trial of planned cesarean or vaginal delivery for twin pregnancy

Background: Twin birth is associated with a higher risk of adverse perinatal outcomes than singleton birth. It is unclear whether planned cesarean section results in a lower risk of adverse outcomes than planned vaginal delivery in twin pregnancy.\ud \ud Methods: We randomly assigned women between 32 weeks 0 days and 38 weeks 6 days of gestation with twin pregnancy and with the first twin in the cephalic presentation to planned cesarean section or planned vaginal delivery with cesarean only if indicated. Elective delivery was planned between 37 weeks 5 days and 38 weeks 6 days of gestation. The primary outcome was a composite of fetal or neonatal death or serious neonatal morbidity, with the fetus or infant as the unit of analysis for the statistical comparison.\ud \ud Results: A total of 1398 women (2795 fetuses) were randomly assigned to planned cesarean delivery and 1406 women (2812 fetuses) to planned vaginal delivery. The rate of cesarean delivery was 90.7% in the planned-cesarean-delivery group and 43.8% in the planned-vaginal-delivery group. Women in the planned-cesarean-delivery group delivered earlier than did those in the planned-vaginal-delivery group (mean number of days from randomization to delivery, 12.4 vs. 13.3; P = 0.04). There was no significant difference in the composite primary outcome between the planned-cesarean-delivery group and the planned-vaginal-delivery group (2.2% and 1.9%, respectively; odds ratio with planned cesarean delivery, 1.16; 95% confidence interval, 0.77 to 1.74; P = 0.49).\ud \ud Conclusion: In twin pregnancy between 32 weeks 0 days and 38 weeks 6 days of gestation, with the first twin in the cephalic presentation, planned cesarean delivery did not significantly decrease or increase the risk of fetal or neonatal death or serious neonatal morbidity, as compared with planned vaginal delivery