DIFFERENTIATION OF THE GONADS AND INITIATION OF MAMMARY-GLAND AND SCROTUM DEVELOPMENT IN THE BRUSHTAIL POSSUM TRICHOSURUS-VULPECULA (MARSUPIALIA)

The first appearance of the mammary and scrotal primordia and the sexu al differentiation of the gonads of the brushtail possum, Trichosurus vulpecula, are described. Primordial germ cells were first observed, i n fetuses of 7.5 mm crown-rump length, in the gonadal ridges and migra ting up the dorsal mesentery. Mammary primordia were first observed in fetuses of 11 mm, and scrotal primordia in those of 12 mm crown-rump length. These structures were diagnostic of female and male brushtail possums respectively. Processus vaginales and gubernacula showed sexua l dimorphism, being better developed and appearing earlier in males th an in females. Sexual differentiation of the gonads occurred after the appearance of mammary and scrotal primordia, the testes being first r ecognisable in a 14.5-mm fetus and the ovaries postnatally. Birth occu rred between the stages of 14 and 15 mm crown-rump length. These obser vations appear to indicate that the development of mammary and scrotal primordia are not under gonadal hormonal control, but under direct ge netic control, as suggested for the tammar wallaby by previous authors .