Gonadotrophin and progesterone concentrations in placentae of grey seals (Halichoerus grypus)

Chorionic gonadotrophin (CG) measured by bioassay and progesterone by immunoassay were present in the placentae of grey seals (Halichoerus grypus) from about 20 days after implantation until parturition. During this period a small and significant decrease in the concentration and a large increase in the total amount of placental CG occurred. The biological activity of seal placental CG was neutralized with an anti-hCG serum. There was a correlation between fetal length and the total amount of progesterone in the placenta. The corpus luteum persisted throughout pregnancy and there was a significant correlation (P < 0.001) between CL diameter and the weight of the ipsilateral ovary. Hypertrophy of fetal testes and ovaries was observed and adult gonad size was reached at about the time of parturition. We suggest that placental CG is the hormone responsible for the precocious enlargement of fetal gonads in grey seals.