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



by distinct sexes, without the slightest trace in their structure of a respiratory ap- 

 paratus. 



As in the Bird, however, so in the ordinary Mammalia there is a period when a tem- 

 porary respiratory organ is essential to the continuance of intra-uterine existence. 

 But previous to the production of this, it is now ascertained that every mammal is 

 developed by means of omphalo-mesenteric vessels ; and it is interesting to observe, 

 that the membrane over which they are spread, or the umbilical vesicle, is largest and 

 most permanent in the order Glires, which, next to the Marsupiata, exhibit the most 

 striking affinities to Birds, and to which the Marsupiata pass by the most natural 

 and unbroken transition. 



The temporary organ for the elimination of the effete particles of the mammiferous 

 embryo is at first, as in birds, an allantois, which, extending from the lower part of 

 the intestine, is developed in different proportions in the different orders ; but the 

 umbilical vessels coextended with it rapidly seek a more intimate contact with the 

 vascular surface of the womb, and accordingly proceed to organize the chorion, 

 shooting out into villi, either extended over the whole surface, as in the Mare; or dis- 

 posed in circumscribed tufts, as in the Ruminants; or limited to one particular spot, as 

 in the Human subject and in all the ordinary Unguiculate quadrupeds. 



It would appear, indeed, from the examination of the small mammary foetuses of 

 the Kangaroo, Petaurus, and Phalangista before mentioned, that an allantois and 

 umbilical arteries are developed at a later period of gestation than the uterine foetus 

 here described had arrived at. None of the above specimens, however, presented any 

 trace of an umbilical vein extending to the liver, and I therefore regard it as impro- 

 bable that the umbilical arteries spread over the chorion to organize a placenta. As, 

 moreover, the uterine foetus preserved by Mr. Bennett had attained two thirds of 

 its full size as such, it is not likely that the allantois would afterwards be developed 

 further than to serve as a receptacle of urine ; and it is interesting to observe that it 

 is arrested at this point in the Batrachian Reptiles. So far, therefore, as the evidence 

 of this specimen goes, it may be concluded that the chorion does not become organ 

 ized, and that the Marsupiata are essentially ovovivi parous. 



In regard to their foetal membranes and appendages they resemble the embryos of 

 the Fowl and ordinary Mammalia at the earliest stages of development ; but when 

 growth has extended so far as to render respiration by an umbilical vesicle no longer 

 adequate to the continuance of intra-uterine existence, instead of a temporary struc- 

 ture being specially organized for that purpose, the lungs are precociously developed, 

 and the embryo of the marsupial quadruped quits the uterus prematurely, and breathes 

 the air. 



§ 2. On the Mammary Foetus of the Marsupiata. 



The period of uterine gestation in the Virginian Opossum, according to Barton, is 

 twenty-six days ; the accuracy of which observation is confirmed by Rengger's expe- 



