Mice of strain A2G, living and breeding in an environment at -3°C., have a resting metabolic rate about four times that of controls at 210 C. The corresponding figure for mice of strain C57BL is more than three times that of the controls. In the warm environment C57BL mice have a higher metabolic rate than A2G mice, but in the cold the A2G mice have the higher rate.Abdominal adipose tissue per unit body weight is much lower in the cold in strains A2G and A, but not in strain C57BL. Total body fat in strain A2G is much lower in the cold than in the warm, but is not significantly different in strain C57BL. Strain differences in fat content parallel differences in growth: C57BL mice are lighter than A2G and A mice in the warm but not in the cold; A2G and A mice are lighter in the cold than in the warm, while C57BL mice show no significant difference in the two temperatures.In adipose tissue and body fat, mice of strains A2G and A, but not C57BL, are more variable in the warm than in the cold. A hypothesis is advanced to account for this.

Breeding stocks of mice of strain 0A2G have been studied at environmental temperatures of \m=-\3\s=deg\and 21\s=deg\C. The mean age of opening of the vagina was 33 days at \ m=-\ 3\ s=deg\ C, 26 days at 21\s=deg\C. The mean body weight at opening was 13 g in both temperatures. The vaginal smear of typical oestrus appeared at a mean age of 61 days at \ m=-\ 3\ s=deg\ C, 38 days at 21\ s=deg\ C; it was preceded by variable numbers of anomalous smears containing squamous cells. The mean length of the oestrous cycle was 8\m=.\5 days at \m=-\3\s=deg\ C, 4\m=.\8 days at 21\s=deg\C. Females transferred from 21\s=deg\to \m=-\3\s=deg\C had longer cycles at first, but tended to return to normal after some months. The interval between parturitions had two modes, at about 3 and 6 weeks respectively: most intervals were around 6 weeks at \m=-\3\ s=deg\ C, 3 weeks at 21\s=deg\C. There was evidence of a negative correlation between the numbers weaned in successive parturitions, when the interval between parturitions was near the minimum. The slowing of the reproductive cycle at \m=-\ 3\ s=deg\ C may be attributed to the prior demands of catabolism; but this does not account for the recovery of the mice transferred from warm to cold.Mice of several strains have been bred for many generations in an environment kept at -3°C , and compared with parallel stocks at 21°C. The mice in the cold environ¬ ment have a much increased metabolic rate and a higher mortality between birth and weaning at 3 weeks; they also build better nests and begin to breed later. Other effects vary with the strain of mouse. The evidence for these statements has been given by Barnett [1956], Barnett, Coleman & Manly [1959], and by Barnett & Manly [1956. Here we are concerned with the effect of cold on the opening of the vagina, the oestrous cycle and the interval between parturitions; and we show that in mice of strain A 2G all these are markedly influenced by a cold environment. METHODThe mice used were of strain A2G, supplied by Glaxo Laboratories in 1953. All pairs used for the maintenance of the stock, or for studies of reproduction, were littermates. They were kept as permanent breeding stocks in each of two constant tem¬ perature rooms, one at 21°C and the other at -3°C. All the mice reported on here had been conceived, born and reared in one of these rooms. All mice had cotton wool in which to nest. Details of the conditions in which they were kept have been given by Barnett [1956]. The observations on vaginal opening and oestrous cycles were made on mice of the eleventh to seventeenth generations in the warm room and of the eighth and ninth generations in the cold.Members of mated pairs were kept always together. Young were removed from

The effect of hybridity on reproduction in mice has been studied in two environmental temperatures: 21° C. (‘normal’) and −3° C. (unfavourable). Mice of inbred strains A2G and C57BL are maintained as permanent breeding stocks in each of the two temperatures. In each temperature the two strains were crossed, and the reproduction of F1 pairs, to the age of 28 weeks, compared with that of the parent strains.At 21° C. the expected superior fertility of the F1 mice was found: the number of litters produced was not affected, but there were more young produced per pair. The number of young per pair reared to 3 weeks was about twice the mean of the parent strains at the same temperature.At −3° C. the difference was greater. The F1 pairs produced more and larger litters than the parent strains, and deaths between birth and weaning at 3 weeks were few. The number of young per pair reared to 3 weeks was nearly five times the mean of the parent strains. Part of the superiority of the F1 mice, at both temperatures, was due to the fact that they began to breed earlier.Despite the larger litters produced by the F1 pairs, at 21° C. the mean weight of the F2 young at the age of 3 weeks was intermediate between those of the parent strains. At −3° C. it was the same as that of the heavier parent strain, namely, A2G.As a further test of resistance to cold, F1 mice born in the warm environment were transferred to the cold at the age of 22 days and there placed each alone in a cage, with nesting material. In these conditions they had a higher survival rate than the young of either of the parent strains. They also grew faster than the A2G mice.The F1 mice were not only more fertile than the inbred mice, but also more uniform in breeding performance. This difference was especially marked in the less favourable environment.These observations are in conformity with the view that heterosis is a consequence of heterozygosis; and that it depends on an enhanced ability to withstand disturbances of developmental and physiological equilibria.

Mice of four strains were reared at 21°C. and then transferred to ‐3°C., in individual cages with nesting material. Controls were kept in similar conditions at 21°C. Many of the mice of two A strains died when stressed by cold at the age of 3 weeks, but none at 5 or 12 weeks. Many C57BL and GFF mice died when stressed at 5 weeks. Death usually occurred during the first 3 days, often on the first day. All stressed mice lost weight, especially in the first 2 days; if they survived, they made up the lost weight but they grew more slowly than the controls at all stages. Mice reared at ‐3°C. and stressed in that temperature when 3 weeks old had a lower mortality than those transferred from 21°C., and survivors did not lose weight. Mice reared at ‐3°C. and transferred to 21°C. at 3 weeks, although initially lighter, grew quickly and after 1 week were as heavy as the controls. Three‐week‐old mice stressed at ‐3°C. lost liver glycogen quickly whether they had been reared at 21°C. or ‐3°C. This was not due to a failure to eat, and there was no evidence that death resulted from inability to mobilize carbohydrate reserves.

It is well known that in aqueous solution most metal ions (e.g., Be2+, Al3+, Fe3+) form progressively less stable complexes with fluoride, chloride, bromide, and iodide. For others (e.g,, Ag+, Hg2+, Tl3+), this stability sequence is reversed. Ions of these types were first called class (a) and class (b) acceptors, respectively (1). Pearson (2) extended and restated the above observations in the form: hard (Lewis) acids (e.g., Be2+) prefer to combine with hard bases (e.g., F-), and soft acids with soft bases (e.g., I"). Hard acids and bases are described as small, less polarizable species, and soft acids and bases as large, more polarizable species. It is often taken for granted that, in contrast to class (a) or hard cations, class (b) or soft cations form stronger bonds to the larger halide ions than to fluoride. This is not necessarily so: in an aqueous equilibrium