We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in a YBa2Cu3O7 single crystal at fields higher than previously reported. For field directions close to perpendicular to the CuO2 planes, we find that the flux lattice structure changes smoothly from a distorted triangular coordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent d-wave theories but is 45 degrees from that recently observed in La(1.83)Sr(0.17)CuO(4+delta).

No abstract

SUMMARY The oocytes in the ovaries of virgin and multiparous mice (A, CBA, RIII and CBA × A strains) have been classified and counted. The ages of the virgin mice ranged from 0 (the day of birth) to 933 days and of the breeding mice from 93 to 735 days. In each strain there is a continuous gradual decline in numbers of oocytes which can be expressed by regression equations of the form: log y = a + b (x − [unk]), where y = number of oocytes and x = age in days. The strains differ in the stage of development reached at birth, in the total numbers of oocytes they contain and in the rates at which the oocytes are lost (given by the regression coefficients b). CBA mice lose their oocytes most rapidly and this is the only strain in which the ovary became totally depleted of oocytes long before death. The proportion of oocytes classified as normal, on histological grounds, falls during the first few weeks of life to a figure of 50–60%, around which it remains throughout the rest of life. The strain with the highest proportion of normal oocytes was the one in which the decline in total numbers occurred most slowly. Breeding has no significant effect on the rate at which the total number of oocytes declines. Figures for the levels of fertility (in terms of litter size at birth) throughout the reproductive lifespan follow the accepted pattern of rising productivity for the first few litters followed by a gradual decline until the mice become sterile. The levels of fertility differ between the strains but cannot be related either to the total number of oocytes, the rate at which they decrease, or to a decline in numbers of Graafian follicles or corpora lutea. It is suggested that the explanation for the decline in fertility towards the end of the reproductive lifespan is more likely to be found in defects in the hormonal control of the ovary or in the uterine environment rather than in the loss of oocytes per se.

Differential counts of follicles of different sizes in the ovaries of intact and hypophysectomized strain A mice of various ages have been analysed statistically. The influence of age on the number of follicles in each size group is described. The number of small follicles in the ovarian pool declined exponentially, whereas numbers of follicles in other groups displayed two peaks, one during immaturity and the other in adult life. These appeared sequentially in follicles in successive groups. The proportion of follicles growing and dying was estimated and age dependent fluctuations in the time spent by follicles in each group were demonstrated. Further calculations then determined the mean inflow and outflow of follicles to and from the various groups throughout the lifespan. Hypophysectomy during post-puberty decreased only slightly the utilization of small follicles from the pool and did not prevent their subsequent development to the two-layered stage. Many follicles then died at this stage of development, indicating that the initial main site of gonadotrophic action may be on medium sized follicles with two layers of granulosa cells.

1. A surgical technique for the orthotopic transplantation of the ovaries in mice is described and its immediate and long-term difficulties are discussed.2. The shortened reproductive lifespan of mice which have received such grafts is due primarily to the loss of oocytes during the immediate postoperative period before a new blood supply has developed. Figures are given for the loss, which may extend to between a half and three\x=req-\ quarters of the total complement of oocytes.