564 MR. OWEN ON THE OVA OF THE ORNITHORHYNCHUS PARADOXUS. 



the oviduct. But the yolk in Birds and oviparous Reptiles is invariably the product 

 of the ovary, and derives no appreciable increase from the secretions of the efferent 

 tube, which supply only the albuminous part of the egg, or the material for the first 

 formation of the chick. If, therefore, the gestation of the Ornithorhynchus termi- 

 nates by the exclusion of an egg, as in the Bird or Tortoise, the preparatory steps in 

 the formation of the ovum are widely different, for the parts concerned manifest the 

 essential characters of the mammiferous type, and the germ itself has a correspond- 

 ing structure. 



These facts, it is agreeable to find, are in exact accordance with the now ascertained 

 functions of the abdominal glands ; for since the yolk in the Bird, besides its uses in 

 the course of the foetal development, is intended as an after-substitute for a mammary 

 secretion, remaining, as it does, but little diminished at the close of incubation, it 

 might have been concluded, from a priori physiological deduction, that the Ornitho- 

 rhynchus, in which no such substitute is required, would approximate the other 

 Mammalia in the small size of the ovarian ovum. 



The nature or amount of subsequent deviations from a true viviparous generation, 

 can be determined only by future examinations of more advanced ova. From the 

 structure of the cortical membrane it is probable that they do not become organized, 

 and that the Monotremata, like the Marsupiata, are essentially ovoviviparous. Since, 

 however, the female Ornithorhynchus has no tegumentary pouch to protect a prema- 

 turely born offspring, it must be presumed that the foetus acquires greater propor- 

 tional bulk* and more mature strength by a longer continuance within the uterus. 

 In this case it may be doubted whether the vitelline vesicle will sufl[ice for nourish- 

 ment and respiration through the whole period of development, and the allantois and 

 umbilical vessels will probably be more or less developed for that purpose. 



The means of prosecuting this inquiry are the more likely to be afforded, since, 

 through the exertions of Mr. Bennett, the period when the pregnant female may be 

 procured is now ascertained. Had not a specimen supposed to be in this condition, 

 which my friend had preserved alive, unfortunately escaped from its confinement, 

 he would, there is little doubt, have ascertained the true nature of the generative pro- 

 duct, and the probable duration of gestation. 



With reference to the latter point, Mr. Bennett observes, that two months after 

 the capture of the female specimen with the smallest ova, viz. on the 8th of Decem- 

 ber 1832, he succeeded in laying open one of the burrows of the Ornithorhynchi, on 

 the banks of the Murrumbidgee River, in which three living young ones were found : 

 they were naked, and measured only one inch and seven eighths in length, and he 

 considers them to have been recently brought forth. Not having any means of pre- 

 serving these specimens, and being at a great distance from Sidney, they were lost. 



* In reference to this point it may be observed, that the kidneys are not lodged low down in the pelvis, as 

 in the true Ovipara, but occupy the position characteristic of the mammiferous type of structure, which allows 

 free space for the enlargement of the uterus during pregnancy. 



