Mammary Organs.—In the young Marsu pial, as Mr. Morgan was the first to observe in the Kangaroo,. the nipples are not visible, but are indicated by the orifices of a kind of cutaneous preputial sheath in which they are concealed. M. Laurent has noticed a similar condition of the nipples in a mam mary foetus of an Opossum and a Perameles. I have also observed it in the marnary_ fmtus of a Petaurist and Dasyure : it is doubtless, therefore, common to all Marsupials.
Once naturally protruded and the preputial sheath everted, the nipples, in the Kangaroo at least, continue external. They are longer and more slender than in other quadrupeds, and when in use generally present a terminal expansion (fig. 142, d). This part lies in a deep longitudinal fossa on the dorsum of the tongue (a, fig. 142); and the originally wide mouth of the uterine foetus is changed to a long tubular cavity, with a terminal sub circular or triangular aperture just large enough to admit the nipple, to which the young Mar supial thus very firmly adheres.
In the Phascogale, in which the nipples are relatively larger than usual, and of a subcom pressed clavate form, the young, when grown too large to be carried in the pouch, are dragged along by.the mother, if she be pursued, hanging by the nipples.
The number of nipples bears relation in the marsupial, as in the placental Mammalia, to that of the young brought forth at a birth; although from the circumstance of the produce of two gestations being for a short time suckled simultaneously, the nipples are never so few. Thus the uniparous Kangaroo has four nipples; of which the two anterior are generally those in use : the Petaurists, which bring forth two young at a birth, have also four nipples : the Thalycine has four nipples : the multiparous Virginian Opossum has thirteen nipples, six on each side and the thirteenth in the middle. In the Didelphys Opossum there are nine nipples, four on each side and one in the middle. The Didelphys dorsigera has the same number of nipples, although six is the usual number of young at a birth (fig. 143). In the Phas cogale penicillata there are eight nipples ar ranged in a circle. The Perameles nasuta has the same number of nipples arranged in two slightly curved longitudinal rows ; this Mar supial has three or four young at a birth.
The nipple in all the Marsupials is imper forate at the centre ; the milk exudes from six to ten minute orifices arranged round the apex. It increases in size with the growth of the mammary fmtus appended to it.
The mammary gland has the same essential structure as in the ordinary Mammalia; it has no cavity or udder; its chief peculiarity arises from its being embraced by the muscle, already noticed, which has the same origin and course as the cremaster muscle in the male.
Marsupial pouch.—The development of the pouch is in an inverse ratio to that of the uteri and directly as that of the complicated vaginw thus it is rudimental in the Dorsigerons Opos sum, which has the longest uteri and the sim plest vaginae: we may conclude therefore that the young undergo a greater amount of deve lopment in the womb in this and allied species.* soon obliterated, as the scrotum increases in size.
In the male Thylacine the rudimental mar supium is retained, in the form of a broad triangular depression or shallow inverted fold of the abdominal integument, from the middle of which the peduncle of the scrotum is con tinued. In the female the orifice of the capa cious pouch is situated nearer the posterior than the anterior boundary of that receptacle.
In the Kangaroos and Potoroos, which have the shortest uteri and longest vaginal tubes and cul-de-sac, the marsupial pouch is wide and deep. It is composed of a duplicature of the integument, of which the external fold is sup ported by longitudinal fasciculi of the panni culus carnosus converging below to be im planted in the symphysis pubis. The mouth of the sac is closed by a strong cutaneous sphincter muscle. The interior of the pouch is almost naked : a few hairs grow around the nipple: it is lubricated by a brown sebaceous secretion. The mouth of the pouch is directed forwards in most Marsupials : the reversed position in the Perameles and Chwropus, where the mouth is directed towards the vulva, has been already noticed. M. Laurent* has made the interesting observation of the presence of a rudimental pouch in the male mammary fcetus of an Opossum : he could not discern equal traces of the nipples: that of the pouch is A few observations on the claims of the Mar supialia to be regarded as a natural group of ani mals may not inappropriately conclude this ar ticle. Cuvier, in 1816, first separated the mar supial from the other unguiculate quadrupeds, to form a distinct group, which he describes as forming, with the Monotremes, a small collateral chain, all the genera of which, while they are connected together by the peculiarities of the generative system, at the same time correspond in their dentition and diet, some to the Car nivora, others to the Rodentia, and a third tribe to the Edentata. M. de Blainville, in the tables of the Animal Kingdom which he published in the same year, 1816, constituted a distinct sub-class of Cuvier's " small col lateral chain " of mammals, and gave to the sub-class the name of Didelphes in antithesis to that of Monodelphes, by which he distin guished the Placental Mammalia.