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The differences between fetal deep body temperature and maternal aortic temperature were measured in 10 New Zealand white rabbits of 22—29 days gestation with thermocouples thrust into the fetuses immediately after the animals had been killed by a warmed overdose of sodium pentobarbital. Fetal temperatures exceeded maternal temperatures by —2.51 + 0.099.X °C, where X is the gestational age in days. There is 95% confidence that fetal temperatures exceeded maternal temperature by at least 0.25 °C at a fetal age of 29 days. Permanently implanted thermocouples in six nonpregnant rabbits showed that the ventral surface of the abdominal cavity is colder than the aorta, and that the dorsal surface is slightly warmer. Fetal heat loss is partly via the umbilical circulation and partly via the fetal body surface. tissue temperatures in rabbits; fetal heat loss Submitted on September 28, 1964

This investigation demonstrated that post-weaning social isolation has a differential effect upon adult dominance in two subspecies of wild deermice, Peromyscus maniculatus bairdii and Peromyscus maniculalus gracilis. Ten male bairdii reared together as a single group from weaning until 91 days of age were significantly more dominant in adulthood than bairdii reared in isolation. There was no difference in adult dominance between the social- and isolate-reared gracilis. Dominance was measured by pairing each social-reared mouse once with every isolate-reared mouse of the same subspecies in a series of 5-min. competitive drinking situations. These results further illustrate that the same form of early social experience can have a differential effect upon organisms of differing genotypes. There was no significant difference in timidity between social- and isolate-reared mice of either subspecies of Peromyscus.

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SUMMARY1. The permeability of the endothelial cell layer of the foetal capillaries of the rabbit placenta was measured by the tracer-dilution method. Isotopically labelled water, urea, chloride and sodium ions, and inulin were used. Radio-iodinated human serum albumin served as reference tracer. The measurements were made in in situ placentas of 27-29 days gestation of which the foetal circulation was perfused with homologous blood.2. The permeability of water was found to be 1-7 ml. min-. g-l, the permeabilities of urea, chloride ions and sodium ions were in the range of 0-28-0-42 ml. min-'. g-and the permeability of inulin was 0-053 ml. min-' . g-. These values may be underestimates, particularly the value of water.3. The approximate maternal blood space, foetal blood space, and extravascular sodium space were determined in rabbit placentas of 27-29 days gestation and found to be 0-16, 0 07, and 0-48 ml./g respectively.4. The permeabilities of the endothelial layer were compared to those published for the entire placenta and found to be far greater. The resistance to diffusion of the endothelial layer is only a small part of the total resistance to diffusion. Most of the resistance is localized in the syncytioand/or cellular trophoblasts.5. It was concluded that lipid insoluble molecules diffuse through the interstitial water spaces of the trophoblast in few, relatively wide channels but that diffusing water can make use of intracellular as well as extracellular paths.