Stimulation of Birth-mechanism in Experimentally Monotocous and Oophorectomized Rats

It was demonstrated recently 1 that ovarian ablation in the rat during the last third of pregnancy need not interrupt this condition, providing the litter being carried has been surgically reduced to one fetus and that the placentae of the fetuses removed have been allowed to remain intact. Under these circumstances, however, the animals fail to deliver and the young, to be obtained alive, must be recovered by operative procedure; retention in utero much beyond the normal time of parturition serves progressively to compromise their viability. It seems improbable that failure of birth is due to the manipulations involved since control animals, receiving similar treatment save for retention of one ovary, will litter at the usual time. In failure of the birth mechanism the rat appears to differ from other species reported to carry to term following upon oophorectomy; the guinea pig, for example, spayed during the latter half of pregnancy, frequently continues to term and to normal delivery; 2 substantially the same situation prevails for the mare 3 and, as is well known, for the human being. 4

The marked rigidity and contraction of the uterus in cases of uninterrupted pregnancy suggest either that it has become insensitive to, or that it lacks, the stimuli involved in the processes of labor. Since oestrin serves as a powerful, and in certain respects a specific, sensitizer of uterine tissue 5 it seemed probable, in the absence of ovaries, that deficiency in this principle might be an underlying factor in failure of birth. The present communication is concerned with the results of attempts at testing this possibility.