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N. P. JOHNSON, M. X. ZARROW, and V. H. DENENBERG

Summary.

Delivery by Caesarean Section (CS) of pregnant rats at 09.00 hours on the 23rd day of gestation reduced the range of times of ovulation after parturition by 18 hr. Ovulation had not occurred by 15 hr after CS but was completed by 18 hr, an interval which is similar to that of ovulation for females of this colony littering at 09.00 hours. Operation on Day 21 or 22 post coitum did not change the day of ovulation (Day 24) but CS on Days 19 or 20 advanced ovulation to Days 22 or 23, respectively. Prevention of normal delivery by ligation of the uterine horns on Day 22 resulted in no females ovulating on Day 24, and few females ovulating on Days 25 and 26. In such females, CS did not increase the number of rats ovulating.

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J. P. HOLLAND, J. M. DORSEY, N. N. HARRIS Jr, and F. L. JOHNSON

Summary.

The effects of l-thyroxine and of surgical thyroidectomy upon the survival of blastocysts have been studied in 200 albino rats. These rats were ovariectomized on the 3rd day of pregnancy, and maintained on progesterone in order to delay implantation. Delay of implantation was confirmed by laparotomy on the 8th day of pregnancy. Subsequent implantation was accomplished by giving 1 μg oestrone daily from the 9th day of pregnancy. At autopsy on the 14th day of pregnancy hyperthyroid rats and hypothyroid rats which had been maintained on daily injections of 2·0 mg progesterone did not differ from their respective control groups in the number of surviving blastocysts. However, hyperthyroid rats which had been maintained on daily injections of 0·4 mg progesterone possessed more implantation sites than controls. Similarly, the hypothyroid rats maintained on daily injections of 0·3 mg progesterone had fewer implantation sites than controls. The experiments suggest that the level of hyperthyroidism tested is beneficial to the maintenance of implantation of delayed blastocysts when low amounts of progesterone are available, while hypothyroidism tends to be detrimental to this process during low progesterone availability.

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P. A. SCHLEIN, M. X. ZARROW, H. A. COHEN, V. H. DENENBERG, and N. P. JOHNSON

Following removal of their olfactory bulbs, virgin female rats consistently attack and kill rat young whereas non-lactating anosmic primiparous animals do not. The present studies were undertaken to compare the effects of bulbectomy on maternal behaviour in the parous and virgin female rat, and to analyse those experiences responsible for the marked behavioural differences towards young seen in these two groups.

Adult female Wistar rats, maintained by random breeding within a closed colony in the laboratory of M. X. Zarrow since 1949 (W/Z strain), were individually housed in 15 × 10 × 7 in. stainless steel cages under a 13-hr light/11-hr dark regimen (lights on at 07.00 hours), and at an ambient temperature of 70 to 74° F. Food and water were freely supplied. The virgin and non-lactating

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Catherine E M Aiken, Peter P L Swoboda, Jeremy N Skepper, and Martin H Johnson

After fertilization, the mammalian conceptus undergoes cleavage, a process of cell proliferation in the absence of interphase growth. It is not known when cleavage ends and gives way to fully replicative cell cycles with a stable nucleo-cytoplasmic ratio. We have used two-photon excitation and confocal microscopy to measure directly volumes and nucleo-cytoplasmic ratios of whole murine concepti and their individual constituent blastomeres during pre-implantation development up to the early uterine attachment stage (day 5). We show that the total cytoplasmic volume of the conceptus remains constant during pre-implantation development, and that the average nucleo-cytoplasmic ratio increases exponentially throughout the same period. Data from individual blastomeres show that both volume and nucleo-cytoplasmic ratio diverge in the inner and outer subpopulations evident from the 16-cell stage (fifth developmental cycle) onwards. Cells from emergent outer trophoblast populations are larger and have smaller nucleo-cytoplasmic ratios than those from emergent inner pluriblast populations. Moreover, the nucleo-cytoplasmic ratio of the trophoblast appears to be stabilizing, suggesting that for this subpopulation cleavage may end at the 16–32-cell transition. Putative hypoblast and epiblast cell subpopulations within the pluriblast were not distinguishable by volume or nucleo-cytoplasmic ratio. Embryonic stem cell volume was higher than that of either cell subpopulation of expanded blastocysts, and their nucleo-cytoplasmic ratio was similar to that of trophoblast cells.