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



an adherence to the parietes of the uterus. On the sixth day, when the ova of the 

 Rabbit nearly correspond in size to the smallest ova of the Ornithorhynchus above 

 described, they are equally devoid of any adherence to the uterine walls. As, however, 

 the differences between the ova of these animals are so obviously manifested in the 

 greater strength of the outer, or cortical membrane, of the ova of the Ornithorhyn- 

 chus, and in the magnitude which they are already known to attain before any di- 

 stinct development of the foetus can be perceived, there can be little doubt that the 

 generation of this species proportionately approximates towards the oviparous mode. 



On comparing the ovum of the Rabbit with that of the Bitch, it is seen to attain in 

 the former to a considerably larger size before it contracts an adhesion to the uterus, 

 which appears to have relation to the greater share which the umbilical vesicle has 

 in the development of the embryo ; since in the Kangaroo, in which the umbilical 

 vesicle fulfills the functions of the placenta, the chorion remains unattached to the 

 uterus, and unvascular when the foetus is almost fully formed. And as the quantity 

 of vitelline granules accumulated in an ovum is indicative of the size and persistence 

 of the umbilical or vitelline vesicle, we may infer that, in the Ornithorhynchus, the 

 latter will play an important part in the development of the embryo. 



The changes which the impregnated uteri of the Ornithorhynchus had undergone, 

 as compared with the same part in the quiescent state, were greater than those which 

 have been observed to take place in the Kangaroo. The uterus containing the two 

 smallest-sized ova measured seven lines in diameter, but was much firmer and denser 

 than in the unimpregnated' specimens ; and having also increased in length, was 

 thrown into more abrupt curves on either side of the ovarian ligament. The uterus 

 which had contained the largest ovum measured an inch in diameter ; and that 

 containing those of the second size was of nearly the same size (Plate XXV. fig. 2.). 

 The right uterus in all the specimens had become sympathetically affected, being 

 firmer in texture and thicker in its coats. 



The parietes of the impregnated uteri were from three to four lines in thickness ; 

 an increase which was principally occasioned by the extension of small vascular 

 folds between the fibrous and internal coats, which were so placed at right angles 

 to these tunics as to present an appearance very similar to that of the second cavity 

 of the stomach of the Porpesse. The fibrous coat was slightly thickened near the 

 cervix, and the serous covering was separated from it by the ramifications of nu- 

 merous large and tortuous uterine vessels. 



There was not the slightest trace of a decidual or adventitious membrane in the 

 cavity of the womb ; and especial attention was directed to this circumstance in 

 consequence of the office assigned to it in a recent work*, as ministering support 

 to the ova in the higher Mammalia, at a period when, like those of the Ornitho- 

 rhynchus, they have no attachment to the uterine parietes-f-. 



* Breschet, Etudes de I'CEuf Humain. 



t In the recent specimens Mr. Bennett noticed besides the ova only a " moisture" in the uterus. 



