The developmental changes in the placenta of the guinea-pig.

The placental disc and the yolk sac endoderm of intact guinea-pigs from day 14 of gestation to term, and of ovariectomized and fetectomized animals at 35 days of pregnancy, were observed by histological means. The chorionic giant cells were the first to attain maximal development (between days 17-35) and they had begun to degenerate by day 40. The spongy zone syncytiotrophoblast was well established and vascularized by day 18, and the maximal size of the zone was maintained between days 30 and 64. The spongy zone then dwindled in size just before parturition. Labyrinthine tissue appeared on day 18, but rapid growth and complete vascularization was not attained until as late as day 40. However, its continued increase in size almost up to term closely paralleled the growth of the fetus. If the placenta secretes progesterone from day 18 to parturition, the spongy zone is the only tissue sufficiently well established to carry out this activity throughout this period. The compensatory hypertrophy of the spongy zone syncytiotrophoblast in 35 day pregnant animals after ovariectomy and fetectomy supports the hypothesis that the spongy tissue secretes progesterone.