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



in length, and two in diameter, with the margins of the orifice, through which the 

 ovum and granular substance had passed, everted, with a slight contraction, resem- 

 bling the neck of a flask, below the aperture. On compressing these ovisacs, small 

 portions of coagulated substance escaped. When longitudinally divided, they were 

 found to consist of the same parts as the ovisac before impregnation, with the ex- 

 ception of the granular contents and granular stratum ; but the theca, or innermost 

 parietes of the sac, was much thickened, and encroached irregularly upon the empty 

 space, so as to leave only a cylindrical passage to the external opening. 



De Graaf's accurate figure of the corpora lutea in the Rabbit is given at 

 Plate XXV. fig. 1 1 , to show the close correspondence between the two animals in 

 the appearance of these parts ; and their structure is essentially the same. 



The undischarged ovisacs of the left ovary, in the impregnated Ornithorhynchi, were 

 numerous and of a globular form, but did not exceed a line and a half in diameter ; 

 a circumstance which corroborates the opinion before expressed relative to the size 

 of the mature ovisacs. For if these parts really attained, prior to the escape of the 

 ovum, much greater dimensions than those in Plate XXV. fig. 1., it might be expected 

 that the other ovisacs would at least have exhibited some proportional degree of 

 increase. 



The impregnated Ornithorhynchus, in the uterus of which the two smallest-sized 

 ova (Plate XXV. fig. 3.) were found, was shot on the evening of the 6th of October 

 1832, in the Yas River, Murray County, New South Wales. These ova were of a 

 semitransparent white colour when recent, but had lost that appearance when exa- 

 mined at the Museum, to which they had been transmitted, in situ, with the uterus 

 and surrounding parts well preserved in spirits. The ova were situated at the upper 

 part of the left uterus, and at the distance of about a line from each other. Each 

 ovum was spherical in form, and measured two lines and a half in diameter : they 

 were of a deep yellow colour, with a smooth and polished surface, and had not the 

 slightest adherence to the uterine parietes. 



The specimen containing the two ova next in size (Plate XXV. figg. 2 and 4.) was 

 shot in the same locality on the 7tli of October. These ova measured each three 

 lines in diameter, and were situated a little below the middle of the left uterus : they 

 were of a spherical form, but had evidently been slightly compressed in the uterine 

 cavity. They were of a lighter colour than the preceding ; a circumstance which 

 was specially evident at the ui)perpart, from the subsidence of the contained vitelline 

 mass. Externally they were smooth, and rolled freely out of the position where 

 they were lodged, like those of the preceding specimen. 



The third specimen, in the uterus of which the largest ovum was contained, was 

 shot on the evening on which the first specimen was obtained. This ovum had the 

 same spherical form, smooth exterior surface, and freedom from connexion with the 

 uterus, as the preceding, but was of a much lighter colour, owing to the increased 

 quantity of its fluid contents, to which its greater size was chiefly attributable. It 



