MR. OWEN ON THE GENERATION OF THE MARSUPIAL ANIMALS. 341 



Actual observation can alone, however, be relied upon to establish satisfactorily the 

 precise relations subsisting between the Marsupiata and ordinary Mammalia at the 

 first stages of their development ; but so far as analogous reasons can be deduced 

 from observations on the structures immediately concerned, there appears no ground 

 for concluding that any material difference exists in the formation of the ovum in the 

 ovary, or the condition in which it arrives at the uterus. But in the Kangaroo the 

 uterus is evidently destined, from the great development of the lining membrane, 

 to afford an abundant secretion for the increase of the ovum after it has passed 

 into that cavity ; and the chorion, when two thirds of gestation have elapsed, still 

 manifests the same condition as in the earliest period of the ovum in ordinary Mam- 

 malia. No villi have been put forth from its external surface, no adhesion has taken 

 place between it and the inner membrane of the uterus, nor does it appear to have 

 been organized in any part so as to act as a placenta. Granting, therefore, that the 

 membrane organized by the omphalo-mesenteric vessels is an adequate medium for 

 the transmission of the nutrient material to the embryo, it still remains to be deter- 

 mined how its respiration is effected. It is, however, very probable, that notwith- 

 standing the interposition of the chorion, a chemical combination does take place 

 between the carbon of the foetal blood distributed over the widely extended umbi- 

 lical vesicle, and the oxygen of the maternal arterial blood distributed over the highly 

 vascular lining membrane of the uterus ; and this interchange may be sufficient for 

 the purposes of a foetus so imperfect, and during an uterine existence so peculiarly 

 brief, as in the Kangaroo. 



In the ova of Fishes, the vascular membrane expanded over the yolk, not being se- 

 parated from the membrana corticalis by an intervening mass of albumen, suffices for 

 respiration as well as nutrition, until the permanent respiratory organs, the gills, are 

 sufficiently developed. And in the higher Reptiles and Birds, the temporary structure 

 superadded to the vascular covering of the yolk, for the more express purpose of elimi- 

 nating the effete particles of the growing embryo, does not begin to expand until a late 

 stage of formation. 



In the Hunterian series of the incubation of the Gosling, the development of the 

 embryo is seen to have advanced to the formation of the head and eyes, and to the 

 distinct production of the four extremities, whilst the allantois is yet a small vesicle 

 protruding at the posterior extremity of the abdomen ; and until this membrane, by 

 its rapid increase and the coextension of the umbilical vessels, has attained to, and 

 spread itself over, the inner surface of the shell, it is still more difficult to explain 

 the mode in which respiration is effected in the embryo of the Bird than in that of 

 the Kangaroo. 



But limited as these embryos are in their vital actions to that of simple growth, a 

 more perfect means of respiration would seem unnecessary ; and among the inferior 

 animals, the Entozoa exhibit to us beings totally excluded from the atmosphere, yet 

 enjoying still greater powers of action, and in the Nematoidean order, even generating 



MDCCCXXXIV. 2 Y 



