Monotreme fetal membranes

Short-beaked Echidna (Tachyglossus aculeatus) Barossa Valley,
South Australia. It has tucked in its beak at the end by the tuft of grass

The three species of echidna and the duck-billed platypus constitute the order Monotremata. They produce milk and thus are mammals but they lay eggs. The aborigines knew this as reported by Caley in 1803 but their accounts were distrusted. It was not until 1884 that Caldwell could send his famous cable to the British Association: " Monotremes oviparous, ovum meroblastic."

Drawing of the echidna by Charles-Alexandre Lesueur 1802-3

The first description of an echidna, in 1792, was by Captain William Bligh of Bounty fame. The drawing above was made during the French Baudin expedition in 1802-3.

Because they lay eggs, one might suppose monotremes to lack a placenta. Yet two-thirds of embryonic development takes place in the uterus and the embryo is nourished in part by endometrial secretions. These are taken up by the yolk sac through the egg shell membrane, which is porous and able to stretch as the embryo grows in size. This state of affairs is best described as matrotrophy (explained here), although it has been argued that the yolk sac of monotremes ought to be regarded as a placenta (here).

Baltimore: The John Hopkins University Press 2004
ISBN 0-8018-8052-1 (pbk.) 

 For a fascinating and well illustrated account of early work on the embryology of monotremes, the above book can be recommended.