Prof. Owen on the Reproduction of the Opossum. 327 



highly valuable additions to the physiology of the marsupial oeco- 

 nomy, but I must demur to the hypothetical statement, " it is not 

 to be believed that a breathing, sanguiferous, digesting mam- 

 mifer can be developed independently of a placenta." The young 

 Viper is born alive, breathing, circulating its blood, capable of 

 digestion, and was developed independently of a placenta. Nay, 

 this body is not even essential to the development of an organism 

 in which respiration and circulation go on so vigorously as to 

 maintain a high degree of temperature, as, for example, in the 

 Bird. The infinite variety, in the works of Creation, of the means 

 by which similar ends are attained, should teach us to subordinate 

 faith in what seems to be a general rule, to observation, wherever 

 observation can be made. One of the most valuable facts in Dr. 

 Meigs' memoir is the determination of the period of gestation in 

 the Didelphys virginiana, viz. from the 18th of February to the 

 7th of March, with probably a range of a few days more or less. 



In order to determine ex visu the nature of the embryonic 

 membranes and appendages, and their degree of correspondence 

 with those of the embryo of the kangaroo, which has no placenta, 

 the female opossums should be sought after in the interval of 

 the dates given by Dr. Meigs, or between the 10th of February 

 and the 10th of March, and the state of the uterus carefully 

 examined. Few series of preparations would be of more im- 

 portance in elucidating the physiology of marsupial generation 

 than the embryos and membranes of the opossum at different 

 stages of its brief intra-uterine life. Whilst trespassing on your 

 columns, permit me to add the following in reference to a former 

 communication. 



My much-esteemed and learned friend Professor Rymer Jones 

 having intimated to me that he had been in the habit of re- 

 garding the odontoid process as the body of the atlas ; which, in 

 fact, was the opinion of Cuvier in respect of the odontoid of the 

 Chelys; I beg, in order to prevent misapprehension, to disclaim 

 any title to originality in the conclusions to which my comparisons 

 of the cervical wedge-bones in recent and extinct Reptilia have led 

 me, beyond the distinction of the body of the atlas into its peri- 

 pheral and central parts, and the recognition of the so-called 

 body of the atlas in Man and Mammalia, as still being a part of 

 such element of the atlas, and the odontoid as the complementary 

 (central) part, and not the representative of the entire body of 

 the atlas. 



My attention being directed more to the question of the ho- 

 mology of the ' wedge-bones' than of the f odontoid/ in the paper 

 which I transmitted to you in August last, and which was favoured 

 by a place in the last number of the f Annals/ I omitted to refer, 

 as I ought to have done, to the passage in the ' Ossemens Fossiles:' 



