A Timetable of Embryonic Development of the Dasyurid Marsupial Antechinus stuartii (Macleay)

Abstract

The rate of embryonic development was investigated in 278 embryos of A. stuartii. Ovulation occurred when the number of epithelial cells in the urine of the female began to fall at the end of oestrus. The gestation period was 27.2 days (so, & 1.83; s~,kO.55; range, 25-31 days). Most births occurred during the 26th, 27th or 28th days. Data from sequentially sampled embryos was used in preparation of a timetable of development over 27 days from fertilization to birth. The eggs spent 24 h or less in the oviduct, where they were fertilized. After entering the uterus, they rapidly underwent the first and second cleavage divisions, and by the start of the 2nd day had reached the four-cell stage. They then appeared to undergo a period of developmental arrest for up to 4 days. Cleavage was resumed by the end of the 5th day at the latest, after which the blastocyst slowly expanded and cell numbers increased, to 1.0 mm diameter and approximately 100-300 cells at the end of the 11th day. (The unilaminar blastocyst was formed between the 20- and 50-cell stages.) A second period of developmental arrest followed, until the end of the 13th or 14th day, and was followed by slow growth, increase in cell number and differentiation so that by the 16th day the embryo had a cell number of 1000-3000, measured 1.3-1.5 mm and possessed a distinct embryonic area. The bilaminar blastocyst was completed by day 20; primitive streak and notochord formation took a further 2 days. By the end of the 22nd day, the embryo possessed a distinct notochord and the mesodermal mantle had passed the equator of the blastocyst. The first somites, the heart rudiment, neural tube, and early stages in amnion development appeared during the 23rd day. The shell had been lost and implantation, which began during the 23rd day, was complete by the end of the 24th day. Organogenesis, from the advanced primitive streak stage without notochord to birth, took 6 days.