The amount of luteal tissue essential for the maintenance of pregnancy has been determined for various laboratory species. From early experiments and those cited below, it seems clear that different amounts of luteal tissue are necessary at different stages of pregnancy if gestation is to continue. In the rabbit, blastocysts fail to expand in the absence of luteal tissue (Corner, 1928; Adams, 1965), but one to two corpora lutea are sufficient for endometrial proliferation (Brouha, 1934; Adams, 1965); whereas two to four corpora lutea are necessary to depress myometrial activity enough to prevent expulsion of the unattached blastocysts (Adams, 1965). Bilateral ovariectomy after implantation terminates pregnancy in the rabbit (Csapo, 1956).
Destruction of the corpora lutea of the rat on Days 3 to 7 of pregnancy or pseudopregnancy leads to
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