MARSUPIALIA

Summary. Macropus eugenii exhibits embryonic diapause after fertiliza-tion at post partum oestrus. The embryo becomes dormant at the blasto-cyst stage in response to the suckling stimulus of a young in the pouch during the breeding season and embryonic diapause continues during the non-breeding season in the absence of the suckling stimulus. Dormant blastocysts resumed development when progesterone was injected daily for 3 days. Normal dormant blastocysts were recovered from animals up to 30 days after complete ovariectomy, and progesterone\x=req-\ induced resumption of development occurred in blastocysts up to 21 days after ovariectomy. The termination of embryonic diapause in progesterone-injected marsupials is discussed in relation to the hor-monal control of delayed implantation in eutherian mammals