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