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XXVI. On the Ova of the Ornithorhynchus paradoxus. By Richard Owen, Esq.^ 

 Assistant Conservator of the Museum of the Royal College of Surgeons in London. 

 Communicated by Sir Anthony Carlisle, F.R.S. 



Received May 15, — Read June 19, 1834. 



JL HE modes of generation of which the ultimate result is the birth of young en- 

 dowed with powers of action and liberated from the foetal coverings, are usually 

 comprehended under the terms viviparous and ovo viviparous. But the processes by 

 which the requisite development of the foetus 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 vivi- 

 parous 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 ex- 

 planations 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 Vertehrata. The 

 reasonableness or necessity of these objections would have been more apparent if the 

 essential dependence of lactation on placental development had first been demon- 

 strated : for with respect to the observations -j- against which they were directed, these 

 were confined to the elucidation of a single disputed and doubtful point in the eco- 

 nomy 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 Ornithorhyn- 

 chus 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. 



In proceeding now to the more immediate consideration of the structure of the 



* Geoffroy St. Hilaire, in the Gazette Medicale de Paris for January and February, 1833 ; Revue Ency- 

 dop^dique for July and August, 1833. 



t Philosophical Transactions, 1832, p. 517. 



