Part IV—Solving the Mystery
How do platypuses reproduce? In 1821, there was a breakthrough when Patrick Hill, a naval surgeon, wrote to the Linnean Society saying he had talked to an Aboriginal elder and “it is a fact well known to them that the animal lays two eggs about the size, shape, and colour of those of a hen; that the female sits for a considerable time on the eggs in a nest which is always found among the reeds on the surface of the water.” 

More importantly, in 1824, the German anatomist Johann Meckel reported that he had found mammary glands in the platypus! They appeared primitive and opened directly onto the skin without any sign of nipples. Monotremes would represent a transitional form between reptiles and mammals. Geoffrey St-Hilaire rejected this view and said the structures described by Meckel couldn’t be mammary glands because the absence of nipples would make feeding difficult with a duck-bill. He stated that the monotremes belonged in their own separate mammalian order, Monotremata.

In 1831, the Hon. Lieutenant Maule, who was stationed in Australia, reported to the Zoological Society of London that he found several nests of platypus with fragments of eggshell and in one nest he found a female and two young. Two weeks later when the female died, he reported: “on skinning her while yet warm, it was observed that milk oozed through the fur on her stomach.” No teats were visible.

Richard Owen, England’s great comparative anatomist, received two baby platypuses from Lieutenant Maule in New South Wales, and determined in 1834 that the suckling infant’s mouth was designed to take milk in the normal manner. In addition, he clearly determined that there was milk in the babies’ stomachs. 

Not until 1844 was the picture clear. The Scottish embryologist, William Caldwell of Cambridge, arrived in Australia and gathered a group of 150 aborigines to search the Burnett River for the elusive monotremes. He shot a platypus in the act of laying eggs: her first egg had been laid and her second was still in the partially dilated mouth of the uterus. He claimed victory. Platypus was oviparous. It laid soft-shelled eggs with large yolks that were gradually absorbed by the growing young, just as in birds and reptiles! In contrast to birds, where the calcified egg does not change in shape or size, the monotreme egg increases in size during its time in the uterus. Its flexible shell is stretched as nutrients are absorbed from the uterus.

1. Were you surprised? Do these discoveries change your view about how to classify the platypus? 
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