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



larger ova ; the affinity, therefore, which the Ornithorhynchus and some other Mar- 

 supial animals manifest in this particular to the class of Birds, obtains also in those 

 orders of the more normal Mammalia, of which the foetus is characterized by the 

 magnitude and persistence of the vitelline or umbilical vesicle. 



The structure of the ovary of the Ornithorhynchus exhibits all the essential cha- 

 racters of the mammiferous type of this organ ; its fibrous coat is strong and in- 

 elastic, and the cellular substance in which the ovisacs are imbedded is dense, and 

 cannot be stretched without much force. On making a section through an ovisac or 

 Graafian follicle, it is found to be implanted more or less deeply in condensed layers 

 of this cellular substance, to which its theca closely adheres. The innermost layer of 

 the theca is less vascular, thinner, and smoother on the inner surface than in the cor- 

 responding coat in the human ovisac. 



In the Ornithorhynchus the theca of the expanded ovisac at the most prominent 

 part is very thin and transparent ; and the capsule of the ovary is either wanting, or 

 blended with its peritoneal covering, which is slightly protruded, as though the 

 pressure of the contained follicles had stimulated the absorbents to remove the re- 

 sisting laminae, as in the progress of an abscess to the surface of the skin. In the 

 true Ovipara, as in the Fowl and Tortoise, &c., this partial thinning of the capsula 

 ovarii surrounding the mature ovarian ovum, is less perceptible on account of its 

 general tenuity ; and the part where the ovum is about to escape is indicated chiefly 

 by the extremities of the vessels, which converge to it from all parts of the capsule ; 

 it is also of a linear form ; while in the Ornithorhynchus and other Mammalia it is 

 a circular protuberance. 



The contents of the ovisacs of the Ornithorhynchus consist of minute granules, 

 which in the larger ovisacs are applied in a condensed state to the inner surface 

 of the containing membrane, and there form a granular stratum. I have opened 

 with great care many ovisacs of the Ornithorhynchus, of different sizes, under the 

 microscope, separating with the point of a needle the clusters of granules as they 

 escaped, and have examined the inner surface of the capsule, especially opposite the 

 mammillary projection, but never succeeded in detecting the vesicle described by 

 PuRKiNJE and Von Baer as existing in the ovarian ova of other animals : the long 

 maceration of the parts in spirit may, however, have destroyed this delicate but 

 essential part of the ovum ; and the coagulation of the albumen, which is mixed with 

 the granules, adds greatly to the difficulty of this delicate investigation. 



The contents of the larger ovisacs above described varied both in colour and con- 

 sistency : in the smaller of the two taken from the first specimen, the fluid in which 

 the granules were immersed was more abundant, and bore a slight straw-coloured 

 tinge ; in the other ovisac from the same animal, and in those of the Chatham spe- 

 cimen, the granules were more numerous, the contents having a caseous consistency, 

 and being of a deep yellow hue. 



De Graaf describes a similar variety in the consistence and colour of the contents 



