Cleavage in vivo and in culture in the dasyurid marsupial Antechinus stuartii (macleay)

Cleavage in the brown marsupial mouse, Antechinus stuartii , from the zygote to the unilaminar blastocyst, was observed in vivo and in culture and in sections of embryos. The first three divisions were meridional and passed from the yolk pole to the opposite pole. Deutoplasmolysis, resulting in a distinct yolk mass, occurred during the first two divisions. Prior to the third and fourth divisions, the blastomeres elongated and flattened against the zona pellucida. The fourth division was latitudinal and resulted in two histologically distinct rings of eight blastomeres which were at first rounded and then became flattened against the zona. Further divisions and flattening of the blastomeres resulted in a complete unilaminar blastocyst by the time the blastocyst numbered 22 to 30 cells. Some expansion, causing compression of the zona and mucoid layers, occurred before completion of the blastocyst, but most expansion occurred once the blastocyst was complete. No histological differences could be detected between the blastomeres at this stage. Embryos were successfully cultured from the zygote to the rounded four‐cell stage and from the flattened four‐cell stage to the completed unilaminar blastocyst of around 32 cells. Total estimated cleavage times were slower in culture than in vivo, but the relative lengths of time for each division were approximately the same.