MARSUPIALIA. 
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 Didelphys dorsigeraand the 
Petauri, 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 Azare 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 geuera the structure of 
the uteri is as distinct from that of the vagine 
as in the Rodentia. The fibrous or proper 
tunic of the uteri is thicker than that of the 
vaginz, 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 vagine, 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 
er urethro-sexual cavity without making a pro- 
jection; but at the distance of three-fourths of an 
inch from their termination there isa 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 tince ; 
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, pl. vi. fig. 6. 
317 
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 foetus. 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 vagine 
to the uteri would appear to be greater as gesta- 
tion is shorter; the vagine 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 feetal vessels and 
membranes in the genus Didelphys. 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- 
phides and Petauri. 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 
Kangarovs 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 
