MARSUPIALIA. 



317 



with the urethro-sexual passage, but is con- 

 nected with it by cellular membrane only ; 

 as might have been anticipated from the struc- 

 ture presented in the simpler forms of the mar- 

 supial uterus, as in Didelp/n/sdorsigeraand the 

 Petuuri, in which the culs-de-sac do not come 

 into contact with the urethro-sexual passage. 

 The evidence of M. Rengger on the develop- 

 ment of the young and the parturition of the 

 Didelphys Azara is also directly opposed to 

 the theory of a temporary orifice in the mesial 

 cul-de-sac. 



The last form of the marsupial female organs 

 which may be noticed is that which is found in 

 one species at least of the Kangaroo Rat ( Hyp- 

 siprymnus murinus). The type of construction 

 is, however, the same as in the great Kangaroo, 

 but the mesial cul-de-sac of the vagina attains 

 a still greater development ; it not only reaches 

 downwards to the uro-genital passage, but 

 also expands upwards and outwards, dilating 

 into a large chamber, which extends beyond 

 the uteri in every direction. From the sides 

 of this chamber the separated portions of the 

 vagina continue downwards, to terminate, as 

 usual, in the urethro-sexual canal.* 



In all the preceding genera the structure of 

 the uteri is as distinct from that of the vaginae 

 as in the llodentia. The fibrous or proper 

 tunic of the uteri is thicker than that of the 

 vaginae, and the lining membrane is soft and 

 vascular, and disposed in numerous irregular 

 folds, which, in section, give apparently a still 

 greater thickness to the uterine parietes. The 

 whole extent of the vaginae, on the contrary, is 

 lined with a thin layer of cuticle, which is rea- 

 dily detachable, even from the middle cul-de- 

 sac, so generally considered as the corpus uteri 

 in the Kangaroo. 



The inner surface of the culs-de-sac in the 

 Opossum is smooth, but in the lower part of 

 the single cavity in the Kangaroo and Potoroo 

 it presents a reticulate structure. The lining 

 membrane in the lateral canals in all the genera 

 is disposed in regular longitudinal folds, a dis- 

 position which characterizes the true vagina in 

 most of the ordinary quadrupeds. In the 

 Kangaroo, as in the other Marsupialia, the 

 lateral canals communicate with the common 

 or urethro-sexual cavity without making a pro- 

 jection ; but at the distance of three-fourths of an 

 inch from their termination there is a sudden con- 

 traction, with a small valvular projection in each 

 (fig- 138). By those who consider the cul-de- 

 sac and lateral canals as a modification of the 

 corpus uteri, these projections will probably be 

 regarded as severally representing an os tincae ; 

 but as they do not exist in the Opossums and 

 Petaurists, in which there is simply a contrac- 

 tion of the vaginal canals at the corresponding 

 part, and as in both these and the Kan- 

 garoo, the true uteri open in the characteristic 

 valvular manner, as in the Rodentia, without 

 the slightest appearance of a gradual blending 

 with the median cul-de-sac, the valvular struc- 

 ture in the lateral canals cannot be regarded as 



* See Philos. Trans., 1834, pi. vi. fig. 6. 



invalidating the view here adopted of the 

 vaginal nature of the median cul-de-sac, 

 which is supported both by the general tex- 

 ture and connexions of the part in question, 

 as well as by what is now ascertained to be its 

 limited function. Moreover, in the large single 

 vagina of some of the Rodentia, as the Hare, 

 Rabbit, and Paca, there are two corresponding 

 valvular folds of membrane near its commence- 

 ment, a little way above the urethral aperture. 



In endeavouring to trace the purposes an- 

 swered by the different forms of the female 

 marsupial organs above described, considerable 

 difficulty arises from the want of the necessary 

 evidence which would be afforded by the ex- 

 amination of the pregnant uterus in each genus, 

 and by the absence of information as to their 

 respective periods of gestation, and the powers 

 of the new-born fetus. As far, however, as a 

 conclusion can be drawn from the relative pe- 

 riods of gestation in the Kangaroo and Opos- 

 sum, the proportionate capacities of the vaginae 

 to the uteri would appear to be greater as gesta- 

 tion is shorter; the vaginae being thus calculated 

 to present fewer obstacles to the escape of the 

 foetus in proportion to the duration of its uterine 

 existence ; and, consequently, a less capacious 

 and complete external pouch is requisite for its 

 ultimate perfection. From Rengger's descrip- 

 tion of the connexion of the foetal Opossum to 

 the uterus, it might be concluded that the gene- 

 ration in that animal approximated to the true 

 viviparous mode more nearly than it does in the 

 Kangaroo; but the determination of this inte- 

 resting question will require a more exact inves- 

 tigation into the nature of the foetal vessels and 

 membranes in the genus Didtlphys. The im- 

 pregnated uteri of the smaller pouchless Opos- 

 sums of South America would be objects of 

 peculiar interest and value in the present state 

 of the inquiry. 



With respect to the variations of structure 

 in the marsupial female organs, it may also be 

 remarked, that though they are apparently 

 most complicated in the Kangaroos and Pha- 

 langers, yet in reality they deviate from the 

 type of the normal Mammalia in a minor de- 

 gree in these Marsupialia than in the Didel- 

 phtdes and Petuuri. For, the essential differ- 

 ence being a division of the vagina into two 

 canals, we find this bipartition to be most com- 

 plete in the multiparous genera, while in the 

 Kangaroos the division is only partial, and the 

 complexity arises from augmented capacity and 

 extent. It is to be observed, however, that the 

 bipartition of the vaginal canal in the Kanga- 

 roos is not continued from the uterus into the va- 

 gina, leaving its distal extremity single, but com- 

 mences at the urethro-sexual cavity, and is 

 arrested near the uteri, the orifices of which thus 

 open into a common canal. 



The situation of the rudimentary vaginal 

 septum or hymen in the unimpregnated female 

 organs of the placental Mammalia before men- 

 tioned, corresponds with this formation in the 

 Kangaroo ; and in a case where this septum 

 was preternaturally developed in the human 

 subject, it was found to obey the same law of 



