In Didelphys dorsigera the uteri (fig. 139, c, c,) rather exceed the unfolded vagina in length. In most Marsupials the vagina at first descend as if to communicate directly with the urethro-sexual passage ; but in this small Opossum, in which the abdominal pouch consists of two slight longitudinal folds, and the young, as is implied by its trivial name, are transported by the mother on her back, each vaginal tube (c, e, fig. 139,) after em bracing the os tincw (d), is immediately con tinued upwards and outwards, then bends downwards and inwards, and, after a second bend upwards, descends by the side of opposite tube to terminate parallel with the extremity of the urethra (h) in the common or uro-genital passage (f).
In the Petauri, the vagina, when unfolded, are a little longer than the uteri. On examining a specimen of the Pigmy Petaurist which had two very small young in the pouch, I found both the true uteri of three times the diameter of the same in an unimpreguated specimen ; but the vagina were unaltered in size, indi cating that the situation in which gestation takes place in this species is the same as in the Kangaroo. The vagina, after receiving the uteri, descend close together half-way towards the commencement of the urethro-sexual pas sage, but do not communicate together in this part of their course. From the upper part of these culs-de-sac they are continued upwards and outwards, forming a curve, like the han dles of a vase, then descend, converge, and terminate close together, as in the preceding example.
In Dasyurus viverrinus and Didelphys Vir giniana, the mesial culs-de-sac of the vaginas descend to the urethro-sexual passage, and are connected to, but do not communicate with it. The septum dividing them from each other is complete, being composed of two layers which can be separated from each 'other, and which result indeed from the apposition and mutual cohesion of the vagina at this part. In order to reach the common passage, each tube is con tinued outwards from the upper end of the cul de-sac, and forming the usual curve, terminates parallel to the orifice of the urethra. The vagina in the Dasyures are smaller in propor tion to the uteri than in the Virginian Opossum, but of a similar form.
In another species, the Didelphys Opossum of Linnaeus, it would appear from the descrip tion and figures of that the septum of the mesial culs-de-sac of the vagina was im perfect; but it is doubtful whether this inter communication was not the result of parturition, or of an accidental rupture in the specimen ex amined. If it should prove to be a specific difference of structure, it is an approximation to the condition of the female organs in the Phalangers, the Wombat, and the Kangaroos.
In the Marropus major the vagina (fig. 138, c, e') preponderate in size greatly over the uteri (c, c'); and, the septum (e) of the descending cul-de-sac being always more or less incom plete, a single cavity (e) is thus formed, into which both uteri open ; but however imperfect the septum may be, it always intervenes and preserves its original relations to the uterine orifices (d, d).
The foetus has been conjectured to pass into the urethro-sexual cavity by a direct aperture formed after impregnation at the lower blind end of the cul-de-sac, but I have not been able to discover any trace of such a foramen in two kangaroos which had borne young; and be sides, I find that this part of the vagina is not continuous by means of its proper tissue 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 Didelphysdorsigeraaml 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 Azaree 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 vagina: as in the liodentia. The fibrous or proper tunic of the uteri is thicker than that of the vagin, 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.