margin: the nasal cavity then extends backward, and terminates immediately above the larynx, the tip of the epiglottis extending into it, and resting upon the soft palate.

On the middle line of the upper mandible and a little anterior to the nostrils there is a minute fleshy eminence lodged in a slight depression, fig. 601, b. In the smaller specimen this is surrounded by a discontinuous margin of the epidermis, with which substance, therefore, and probably (from the circumstance of its being shed) thickened or horny, the caruncle had been covered. It is a structure of which the upper mandible of the adult presents no trace, and is obviously analogous to the horny knob which is observed on the upper mandible in the foetus of aquatic and gallinaceous Birds. I do not, however, conceive that this structure is necessarily indicative of the mandible's having been applied, under the same circumstances, to overcome a resistance of precisely the same kind as that for which it is designed in the young Birds which possess it. The shell-breaking knob is found in only a part of the class; and although the similar caruncle in the Ornithorhynchus affords a curious additional affinity to the Aves precoces, yet, as all the known history of the ovum points strongly to its ovo-viviparous development, the balance of evidence is still in favour of the young being brought forth alive.

The situation of the eyes, ib. c, was indicated by the convergence of a few wrinkles to one point; but when, even in the larger of the two specimens, these were put upon the stretch, the integument was found entire, and completely shrouding or covering the eyeball anteriorly. The fact is of importance to the question of the mammiferous character of the Ornithorhynchus. For on the supposition of the young animal possessing locomotive faculties, which would enable it like the young gosling, immediately after birth or exclusion, to follow the parent in the water, and there to receive its nutriment (whether mucous or otherwise), the sense of vision ought certainly to be granted to it in order to direct its movements. The privation of this sense, on the contrary, implies a confinement to the nest, and a reception on land of the mammary secretion of the parent. The auditory orifices, ib. d, are situated about a line behind the eyes. The general form of the body and the cartilaginous condition of the bones of the extremities equally militate against the young Ornithorhynchus possessing, at this period of its existence, active powers of swimming or creeping. The head and tail are closely approximated on the ventral aspect, requiring force to pull the body out into a straight line; and the relative quantity of integument on

the back and belly shows that the position necessary for the due progressive motions is unnatural at this stage of growth.

The toes on each of the four feet were completely formed, and terminated by curved, conical, horny claws; but the natatory fold of membrane of the fore foot had not the same proportional extent as in the adult, and the spur of the hind foot did not project beyond its socket in either specimen. In the smaller one, which was a male, it presented the form of an obtuse papilla;

602

while in the larger specimen, although a female, it was more plainly developed and more pointed, fig. 602, f. This circumstance is in exact accordance with the known laws of the development of sexual distinctions, especially of those of secondary importance, such as beards, manes, plumes, horns, tusks, spurs, &c., which do not avail in distinguishing

Hind-foot and spur, young female Ornitho- the sexes till towards the period of

puberty.

In the Echidna hystrix the mammary glands resemble in structure and position those of the Ornithorhynchus : but the ducts, when the gland is functionally developed, open into a small tegumentary pouch, fig. 603, c. The gland, ib. a, is of a flattened, subelliptic form. The lobules converge toward the mesial line, in their course to terminate in the fundus of the pouch. Each lobe is a solid parenchymatous body; the duct is more directly continued from a canal which may be traced about halfway toward the fundus of the lobule; the canal gives off numerous short branches from its circumference, which subdivide and terminate in clusters of subspherical acini' or secerning cellules. The structure is on the same general plan as that of the mammary glands in higher Mammals, but the cellules are proportionally larger. Each gland consists of about 100 long, narrow, flattened lobes, obtusely rounded at their free ends; they are surrounded by a loose capsule of cellular tissue, and lie between a thick 'panniculus carnosus,' adherent to the abdominal integument, ib. d, and the obliquus externus abdominis' muscle, on a plane exterior or lateral' to the pouch. On each side of the abdominal integument, about two inches in advance of the cloaca, and about three inches and a half from the base of the tail, is the aperture, which is longitudinal and directed towards the median line. The skin of the abdomen, where

it begins to be inverted, loses thickness, and at the fundus of the pouch, ib. c, is only half as thick as where it overspreads the abdomen.

I have not hitherto met with any trace or beginning of such abdominal pouches in the various Ornithorhynchi in which I have had occasion to note different phases of the development of the ovaria and mammary glands. A warm-blooded air-breather, compelled to seek its food in water, could not safely carry the progeny

Mammary gland, pouch, and young. Echidna Hystrix. CCCXXII".

it had brought forth in a pocket beneath its body during such quest: all observers have noted the nest-making instinct of the Platypus, and in such temporary and extraneous structures only have the young been hitherto found.

The question remains, whether the marsupial pouches of the Echidna increase with the growth of the young? It is certain that they only commence with the growth or enlargement of the mammary glands preliminary to birth. In the young specimen of female Echidna in which the glands were first discovered' their ducts opened upon a plane surface of the abdominal integument. In a nearly full-grown unimpregnated female there was also a total absence of inflected folds of the integument where the mammary ducts terminate. Some movement, perhaps, of these ducts in connection with the enlargement of the mammary lobes, under the stimulus of preparation for a coming offspring, may, with associated growth of the abdominal integument surrounding the areola, be amongst the physical causes of the first formation of the pouch.

The young Echidna, ib. e, resembles the new-born Kangaroo in the proportions of the limbs to the body, in the inferior size and development of the digits of the hinder pair, and in the feeble indication of eyes or eyelids. But the mouth is proportionally 1 cccxx". p. 179.

wider, and has the form of a transverse slit; it is not circular. Upon the upper lip, in the midline between the two nostrils, is a small protuberance, corresponding to that in the young of the Ornithorhynchus paradoxus, and wanting the cuticle. The tongue is broad and flat, extending to the rictus oris,' but very short in proportion to that of the parent, and of a very different shape. The tail is much shorter than in the young Kangaroo, and shows as much proportional size as in the fullgrown Echidna, in which it is a mere stump concealed by the quills and hair. The head is proportionally longer and more slender in the marsupial fœtus of the Echidna than in that of the Kangaroo, and already, at this early period, foreshows the characteristic elongation and attenuation of that part in the mature animal. The form of the mouth, as a transverse slit, is a good monotrematous character of the young at that period, since in all true or teated marsupials the mouth of the mammary fœtus has a peculiar circular and tubular shape. A scarcely visible linear cicatrix at the middle of the lower part of the abdomen is the sole trace of umbilicus. A bifid, obtuse rudiment of penis or clitoris projects from the fore part of the single urogenital or cloacal aperture, and in advance of the base of the tail-stump. Of the brain, the largest part is the mesencephalon, chiefly consisting of a vesicular condition of the optic lobes.

The fore limbs, in their shortness and breadth, foreshow the characteristics of those of the parent, which may be said, indeed, to retain in this respect the embryonic character with superinduced breadth and strength. The digits have already something of the adult proportions, the first or innermost of the five being the shortest, the others of nearly equal length, but graduating shorter from the third to the fifth; each digit is terminated by a claw: in the hind limb, the second is already the strongest and longest, the rest more rapidly shortening to the fifth than in the fore leg; the innermost, agreeably with the law of closer retention of type in the embryo, though the shortest of the five, is less disproportionally so than in the adult. The young nestles its head and fore-limbs within the marsupial fossa, clinging by its precocious fore claws to the skin or hairs of that part, and imbibing by its broad, slit-shaped mouth the nutritious secretion as it is pressed by the muscles acting upon the gland from the areolar outlets of the ducts.

§ 414. In Marsupialia.-In Marsupials the mammary glands have a more compact form and minutely conglomerate structure than in Monotremes. They are developed on each side the linea

alba at the back of the marsupial depressions, or of the pouch; they are not fewer than two on each side (Macropus, Hypsiprymnus, Phalangista, Petaurus, Phascolarctos, Phascolomys); nor more than thirteen, six on each side and one midway (Didelphys virginiana). The follicles, from the inner surface of which the milk-cells are detached, are cylindrical in shape, th in. in diameter; grouped in clusters of from ten to twenty on short, slender ducts, which enter the sides of larger canals, these uniting to form four or six conical dilatations, from the apices of which as many slender ducts pass to the apex of the nipple. This is peculiar for its length and slenderness when in use; but in the young and virgin Marsupial it is much shorter, and lies at the bottom of an inverted part of the skin of the back of the pouch, which becomes thin and is reflected over the end of the nipple, like the prepuce over the glans penis. The mammary glands enlarge after impregnation, and rapidly a day or two before uterine birth; when, partly from development of the nipple, partly from pressure of the enlarging gland, aided perhaps by the action of its compressor muscle, the sheath is everted and the nipple protruded. The preliminary infolding of the integument provides for the covering of the long nipple, which now is pendant at the back of the pouch. The compressor muscle arises from the ilium between or near to the lower attachment of the internal oblique and transversalis abdominis:' it passes out of the abdominal ring, bends round the marsupial bone, expands as it turns upward and inward behind the pouch to surround partly by carneous, partly by sclerous fibres, the mammary glands, dividing into as many insertions as there are glands of its own side. This muscle (ileo-marsupialis' of Cuvier) is the homotype of the cremaster' in the male (p. 10); and the chief function of the ossification of the internal pillar of the abdominal ring (marsupial bone) is to add the power of the pulley to the compressor of the mammary gland, and effect the requisite change in the course of the contractile fibres. In the pouch of a young Marsupial the nipples are indicated by the inconspicuous orifices of the teat-sheaths. Once naturally protruded and the sheath everted, the nipples continue external. In the Kangaroo, after being some weeks in use, they present a slight terminal expansion, fig. 604, d. This part lies in a deep longitudinal fossa on the dorsum of the tongue, ib. a; 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 Marsupial thus very firmly

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