Embryology of Schoinobates volans (Kerr) (Marsupialia : Petauridae)

Abstract

The embryology of S. volans was studied from 17 embryos ranging from a two-celled ovum to a near-term foetus. The placenta is of the yolk-sac type (choriovitelline) in which the allantois does not reach the chorion. Intrauterine development was similar to that of Didelphis marsupialis virginiana and Trichosurus vulpecula. Forty-nine pouch young were examined, but they did not include any true neonates. The smaller ones showed the precocious development of a number of anatomical features commonly found in other neonatal marsupials. The pronephric and mesonephric tubules were well developed in an embryo of greatest length 3.4 mm. The stage at which the pronephros was completely involuted was not determined. In pouch young of head length 9.5-11.1 mm involution of the mesonephros and proliferation of the metanephros were pronounced. The mesonephros was completely involuted in a pouch young of 18.1 mm head length. The sizes of the mesonephros and metanephros suggested that they function together for some time after birth. It was not possible to sex the young at birth. The gonad was first recognizable as a testis in a pouch young of head length 8.5 mm, and as an ovary at head length 9.3 mm.