Pregnant tammars were ovariectomized during anoestrus or on Day 8 of the reproductive cycle that follows removal of the pouch young. Quiescent and Day-8 blastocysts were transferred to the uteri of anoestrous or Day-8 non-pregnant recipients, some of which were ovariectomized at the time of transfer.
Blastocysts did not develop after ovariectomy performed during anoestrus or after transfer to anoestrous recipients. Pregnancy continued normally after ovariectomy on Day 8 and both quiescent and Day-8 blastocysts developed in the uteri of intact and ovariectomized recipients.
These results indicate that the uterus by Day 8 is competent to nourish an embryo to full term without further ovarian stimulation. They also show that the quiescent blastocyst can resume development without the direct stimulus of ovarian hormones. The hypothesis most in accord with the results is that blastocyst development is inhibited during lactation by a principle of the uterus, which is inactivated during the first 8 days by corpus luteum secretions, and that these secretions also induce the endometrial changes upon which subsequent embryonic development depends.