XXVI. On the ova of the ornithorhynchus paradoxus

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Abstract
The modes of generation of which the ultimate result is the birth of young endowed with powers of action and liberated from the fœtal coverings, are usually comprehended under the terms viviparous and ovoviviparous. But the processes by which the requisite development of the fœtus is effected in the first of these modes, vary remarkably; and so far as they have been investigated in the different orders of Mammalia , to which true viviparous or placental generation is peculiar, a very regular gradation has been traced towards the oviparous or ovoviviparous modes, in which the exterior covering of the ovum never becomes vascular. As lactation has been generally regarded as exclusively associated with a true vivi­parous generation, the arguments adduced in favour of the mammary nature of the abdominal glands of the Ornithorhynchus have been supposed to imply a necessary belief in the accordance of its mode of generation with that of the higher orders of Mammalia . They have consequently been objected to most strenuously by those physiologists who maintain the oviparous nature of this animal: and various ex­planations have been offered, with a view to reconcile the lately ascertained facts respecting the mammary glands with the oviparous theory of the Monotremata , and their supposed position in the natural system as a distinct class of Vertebrata . The reasonableness or necessity of these objections would have been more apparent if the essential dependence of lactation on placental development had first been demon­strated: for with respect to the observations against which they were directed, these were confined to the elucidation of a single disputed and doubtful point in the economy of the Monotremata ; the uterine apparatus being considered so far only as was necessary to determine the correspondence of its periodical changes with those of the mammary glands; while the objections to the oviparity of the Ornithorhyn­chus extended only to the theory which maintained that the ovum was expelled with a calcareous covering, and that embryonic development took place after exclusion by a process of incubation.

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