MR. OWEN ON THE OVA OF THE ORNITHORHYNCHUS PARADOXUS. 561 



measured three lines and a half in diameter, and had been situated in a depression or 

 cell a little below the middle of the left uterus. The lining membrane of the uterus 

 was highly vascular in the recent state in each of the above specimens. 



In all these ova the contents could be seen, tln-ough the cortical or outer mem- 

 brane, to be of two kinds, viz. a greyish subtransparent fluid, arid a yellowish denser 

 mass, which varied in their relative proportions as above mentioned, the denser sub- 

 stance always subsiding to the lowest part of the ovum, whichever way it was turned. 



In the largest ovum, the yellow mass or yolk occupied about one third of its cavity, 

 while in the smallest it constituted four fifths of tlie whole mass. 



The chorion or cortical membrane of these ova (Plate XXV. fig. 6. a.) offered a 

 moderate degree of resistance when torn open with the forceps, and yielded equally 

 in every direction when separated from the yolk, the rent margins curling inwards 

 like the coat of an hydatid. This membrane was of a dull greyish colour, inclining 

 to brown, slightly transparent, and more polished upon its inner thaa upon its outer 

 surface : it resembles the cortical membrane of the ovum of the Salamander, but is 

 of a more delicate texture. Ttie fluid contents occupied the space between the cor- 

 tical and vitelline membranes, a situation aiialogous to that of the albumen in the 

 egg of the Fowl, but had not become coagulated by the action of the spirit in which 

 it had been so long immersed. 



The yellow matter, or yolk, was seen to be invested by its proper capsule (Plate XXV. 

 fig. 6. b.), which, when reflected under the microscope, was found to consist of an 

 extremely thin, scnootli, and transparent outer layer, which I regard as the memhrana 

 vitelli (Plate XXV. fig. 7- d-), with a thicker granular membrane immediately lining 

 it, analogous to the hlastoderma or germinative membrane (Plate XXV. fig. 7- b.) 



The contents of the above investments, or substance of the yolk, consisted of innu- 

 merable minute opake granules, similar in size and regularity of form to those con- 

 tained in the ovarian follicles; and with these granules were mingled larger trans- 

 parent globules of yellow-coloured oil. There was not the slightest trace of chalazae 

 attached to the vitelline membrane, as, from analogy, we should expect to be the case 

 had the ovum been destined to have been perfected by incubation. I was unable to 

 detect any rudiments of the embryo : an opake streak was discernible on one part of 

 the yolk, but not sufficiently definite to be satisfactorily recognised as a cicatricula; 

 it is indeed, probable, from the observation of Lieutenant Maule, that the ova attain a 

 greater size by the imbibition of nutrient material before the lineaments of the foetus 

 become visible. 



The ova of the Rabbit figured by De Graaf the seventh day after the coitus, agree 

 in size with the largest of the ova of the Ornithorhynchus : in Mr. Cruikshank's 

 plate* they are represented somewhat smaller. According to both authors no 

 trace of foetal development is visible at this period ; but it is probable that the 

 formative actions have commenced, as the ova of the Rabbit have now contracted 



* Philosophical Transactions, 1797, Plate IV. p. 204. 



