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theory, suggested by witnessing the actions of the mother after an artificial separation of the Marsupial fœtus, be correct, must be confirmed by actual observation. There is no internal passage from the uterus to the pouch the mouth of the vagina cannot be brought into contact with that of the pouch, either by muscular contraction in the living or by any force of stretching in the dead kangaroo :—as the young was proved by the result of this experiment not to have the power of itself to regain the nipple, a fortiori we may conclude that it could not transfer itself from the vulva to the interior of the pouch and to the apex of the nipple :-the fore-paws of the Kangaroo would not so effectually protect the tender embryo from the external air as the mouth, nor so safely ensure its passage to the pouch, notwithstanding that they are adroitly used in grasping objects, being similar, in respect of the extent and freedom of motion of the digits, to the fore-paws of the Rodents.

After the mother had rested quietly for a short time, we again examined her, but found the young one still detached, moving more vigorously than before. On an examination two days afterwards the marsupium was found empty: the young one had died and had probably been removed by the mother.

Nourishment of the Marsupial Embryo in Utero.

We are at present, says Mr. Owen, ignorant of the changes that take place in the developement of the ovum between the period of impregnation until about the twentieth day of uterine gestation. At this time, in the great Kangaroo (Macropus major,) the uterine fœtus measures eight lines from the mouth to the root of the tail: the mouth is widely open; the tongue large and protruded; the nostrils are small round apertures; the eyeball not yet wholly defended by the palpebral folds; the meatus auditorius externus is not provided with an auricle; the fore-extremities are the largest and strongest; they are each terminated by five well-marked digits; those of the hind legs are not yet developed; the cervical fold of the mucous layer or the branchial fissure is still unenclosed by the integument. The tail is two lines long, thick and strong at the commencement; impressions of the ribs are visible at the sides of the body; the membranous tube of the spinal marrow may be traced along the back between the ununited elements of the vertebral arches; posterior to the umbilical chord there is a small projecting penis, and behind that, on the same prominence, is the anus. This foetus and its appendages were en veloped in a large chorion, puckered up into numerous folds, some of which were insinuated between folds of the vascular lining membrane of the uterus, but the greater portion was collected into a wrinkled mass. The entire ovum was removed without any opposition from a placental or villous adhesion to the uterus. The chorion was extremely thin and lacerable; and upon carefully examining its whole outer surface, no trace of villi or of vessels could be perceived. Detached portions were then placed in the field of a microscope, but without the slightest evidence of vascularity being discernible. The next membrane, whose nature and limits will be presently described, was seen extending from the umbilicus

to the inner surface of the chorion, and was highly vascular. The fœtus was immediately enveloped in a transparent, amnios.

On turning the chorion away from the fœtus, it was found to adhere to the vascular membrane above-mentioned, into which the umbilical stem suddenly expanded. With a slight effort, however, the two membranes could be separated from each other, without laceration, for the extent of an inch; but at this distance from the umbilicus the chorion gave way on every attempt to detach it from the internal vascular membrane, which here was plainly seen to terminate in a well-defined ridge, formed by the

trunk of a blood-vessel.

When the whole of the vascular membrane was spread out, its figure appeared to have been that of a cone, of which the apex was the umbilical chord, and the base the terminal vessel above-mentioned. Three vessels could be distinguished diverging from the umbilical chord and ramifying over it. Two of these trunks contained coagulated blood, and were the immediate continuations of the terminal or marginal vessel; the third was smaller, empty, and evidently the arterial trunk. Besides the extremely numerous ramifications dispersed over this membrane, it differed from the chorion in being of a yellowish tint. The amnios was reflected from the umbilical chord, and formed, as usual, the immediate investment of the fœtus.

The umbilical chord measured two lines in length and one in diameter. It was found to contain the three vessels above-mentioned, with a small loop of intestine; and from the extremity of the latter a filamentary process was continued to the vascular membrane. The margins of the umbilicus or abdominal opening were very strong, offering much resistance to their division. On tracing the contents of the chord into the abdomen, the two larger vessels with coagulated blood were found to unite the common trunk then passed backwards beneath the duodenum, and after being joined by the mesenteric vein, went to the under surface of the liver, where it penetrated that viscus: this was consequently an omphalo-mesenteric or vitelline vein. The artery was a branch of the mesenteric. The membrane, therefore, upon which they ramified, answered to the vitellicle, i. e. the vascular and mucous layers of the germinal membrane, which spreads over the yolk in oviparous animals, and which constitutes the umbilical vesicle of the embryo of ordinary Mammalia. The filamentary pedicle which connected this membrane to the intestine was given off near the end of the ileum, and not continued from the cœcum, the rudiment of which was very evident half a line below the origin of the pedicle.

The small intestine above the pedicle was disposed in five folds. The first from the stomach or duodenum curved over the vitelline vein, and the remaining folds were disposed around both the vitelline vessels. From the cœcum, which was given off from the returning portion of the umbilical loop of the intestine, the large intestine passed backwards to the spine, and was then bent, at a right angle, to go straight down to the anus. The stomach did not present any appearance of the sacculated structure so remarkable in the adult, but had the simple form of a carnivorous stomach. The liver consisted of two equal and symmetrically dis

posed lobes. The vena portæ was formed by the union of the vitelline with the mesenteric, and doubtless the other usual veins, which were, however, too small to be distinctly perceived. The diaphragm was perfectly formed.

The vena cava inferior was joined, above the diaphragm, by the left superior cava, just at its termination in a large right auricle. The ventricles of the heart were completely joined together, and bore the same proportions to each other as in the adult, a perfection of structure which is not observed in the embryos of ordinary Mammalia at a corresponding period of development. The pulmonary artery and aorta were of nearly the same proportionate size as in the adult: the divisions of the pulmonary artery to the lungs were at least double the size of those observable in the embryo of a sheep three inches in length. The ductus arteriosus, on the contrary, was remarkably small. The aorta, prior to forming the descending trunk, dilated into a bulb, from which the carotid and subclavian arteries were given off.

The lungs were of equal size with the heart, being about a line in length, and nearly the same in breadth; they were of a spongy texture and of a red colour, like the veins, from the quantity of blood they contained. This precocious development of the thoracic viscera is an evident provision for the early or premature exercise of the lungs as respiratory organs in this animal: and on account of the simple condition of the alimentary canal, the chest at this period exceeds the abdomen in

size.

The kidneys had the same form and situation as in the adult. The supra-renal glands were half the size of the kidneys.

The testes were situated below the kidneys, and were one-half larger than those glands, the superiority of size depending on their large epididymis, with the adherent remains of the Wolffian body. They continue within the abdomen for six weeks after uterine birth.

At a later period of uterine development, when the fœtus, measured in straight line from the mouth to the root of the tail, is ten lines in length, the urachus expands into a small allantois, of a flattened pyriform figure, and finely wrinkled external surface. This bag insinuates itself between the amnios and chorion, carrying along with it two small hypo-gastric arteries and an umbilical vein, but not establishing by their means an organized and vascular surface of the chorion by which a placental attachment is formed between the ovum and the womb. The allantois depends freely from the end of the umbilical chord, and has no connexion at any part of its circumference with the adjoining membrane. Its office is apparently that of a receptacle of urine.

The chorion

The vitellical or umbilical sac presented the same large proportionate size and vascular structure as in the first described fœtus. which enveloped this fœtus and its appended sacs was adapted to the cavity of the uterus by being disposed in innumerable folds and wrinkles. It did not adhere at any part of its surface to the uterus, but presented a modification not present in the chorion of the earlier fœtus, in being partially organized by the extension of the omphalo-mesentric vessels upon it from the adherent vitellicle. The digits of the hind legs were distinctly formed in this embryo.

No. 87.

13

Anatomical Condition and Development of the Marsupial Mammary

Fætus.

The new-born fœtus of the great Kangaroo does not exceed, as we have shown, one inch in length.

By comparing the new-born Kangaroo with a similarly sized fœtus of a sheep, we find that, although, in the Kangaroo, the ordinary laws of development have been adhered to in the more advanced condition of the anterior part of the body and corresponding extremities, yet that the brain does not present so disproportionate a size; and the same difference is observable in the uterine fœtus of the Kangaroo, even when compared with the same sized embryo of an animal of an inferior class, as the bird. This difference, Mr. Owen apprehends, is owing to the rapidity with which the heart and lungs acquire their adult structure in the Kangaroo, whereby the passage of the purer and more nutritious blood through the foramen ovale and left auricle to the primary branches of the aorta and so to the brain is impeded. The brain, however, of the mammary fœtus, though exhibiting a low degree of development, yet is of a firmer texture than in a similarly sized fœtus of a sheep, and attains its ultimate proportion by a more gradual process of growth.

In a mammary fœtus, one inch and a half in length, the urinary bladder is largely developed, and adheres by its apex to the peritoneum, exactly opposite that part of the abdominal integument where a small linear ridge indicated the previous attachment to the umbilical chord and appendage. There are also minute but distinct traces of umbilical arteries running up the sides of the bladder to this point of attachment. As the urinary bladder becomes afterwards expanded in the abdomen, the peritoneum is gradually, as it were, drawn from this part of the abdominal parietes, forming an anterior ligament of the bladder. In a mammary fœtus of the Kangaroo about a month older than the above, there was, at the superior part of this duplicature a small projecting point from the bladder, like the remains of a urachus; but the fundus, now developed considerably above this point, was covered with a perfectly smooth layer of peritoneum; and it is this modification, he apprehends, which led Hunter to suppose that there was no trace of urachus or umbilical arteries in the fœtuses of the Marsupialia. In the Sloth, the Manis, and the Armadillo, the urachus is continued in the same manner from the middle of the anterior part of the bladder, and not from the fundus.

any corres

In neither of the above fœtuses of the Kangaroo was there ponding trace of umbilical vein, although there was a distinct ligamentum suspensorium hepatis, formed by a duplicature of the peritoneum descending from the diaphragm to the notch lodging the gall-bladder, and not entering, as usual, the fissure to the left of that notch: the allantois is too small, and its function too limited for the preservation of any permanent trace of its peculiar vein.

The small intestines in the mammary fœtus, one inch and a half long, when compared with those of the uterine fœtus above described, were found to have acquired several additional convolutions; the fold to which the umbilical vesicle had been attached was still distinct, but now drawn in to the back of the abdomen. The cæcum was much elongated, but the colon proportionately not more developed than in the uterine fœtus; the sub

sequent modification, therefore, of the large intestines seems evidently destined to complete the digestion of the vegetable food.

The stomach was not sacculated, but the division between the cardiac and middle compartments was more marked than in the uterine fœtus, The liver had now advanced in its development beyond the oviparous form which it presented in the uterine fœtus, the right lobe being subdivided into three. The supra-renal glands bore the same proportionate size to the kidneys. The testes were still larger than the kidneys, and were situated below them, not having yet passed out of the abdomen: this takes place when the mammary fœtus is about three inches long from the nose to the root of the tail. The ductus arteriosus was distinct in the small mammary fœtus, but he could not perceive any trace of the thymus gland. Is this gland unnecessary on account of the precocious development of the lungs ? or because of the small size and gradual growth of the brain? The latter appears the more probable condition of its absence, as in the ovoviviparous classes with small and simple brains the thymus gland is rudimental or of doubtful existence.

Notwithstanding that the new-born Kangaroo possesses greater powers of action than the same sized embryo of a sheep, and approximates more nearly in this respect to the new-born yonng of the rat, yet it is evidently inferior to the latter. For, although it is enabled by the muscular power of its lips to grasp and adhere firmly to the nipple, it seems to be unable to draw sustenance therefrom by its own unaided efforts. The mother, as Professor Geoffroy and Mr. Morgan have shown, is therefore provided with the peculiar adaptation of a muscle (analogous to the cremaster) to the mammary gland, for the evident purpose of injecting the milk from the nipple into the mouth of the adherent fœtus. Now it can scarcely be sup posed that the fœtal efforts of suction should always be coincident with the maternal act of injection: and if at any time this should not be the case, a fatal accident might happen from the milk being forcibly injected into the larynx, unless that aperture were guarded by some special contrivance. Professor Geoffroy first described the modification by which this purpose is effected; and Mr. Hunter appears to have anticipated the necessity for such a structure, for he dissected two small mammary fœtuses of the Kangaroo for the especial purpose of showing the relation of the larynx to the posterior nares. The epiglottis and arytenoid cartilages are elongated and approximated, and the rima glottidis is thus situated at the apex of a cone-shaped larynx, which projects, as in the Cetacea, into the posterior nares, where it is closely embraced by the muscles of the soft palate. The air-passage is thus completely separated from the fauces, and the injected milk passes in a divided stream on either side the larynx to the œsophagus.

Thus aided and protected by modifications of structure, both in the system of the mother and its own, designed with especial reference to each other's peculiar condition, and affording, therefore, the most irrefragable evidence of creative foresight, the small offspring of the Kangaroo continues to increase, from sustenance exclusively derived from the mother, for a period of about eight months. During this period the hind legs and tail assume a great part of their adult proportions; the muzzle elongates; the external ears and eyelids are completed; the hair begins to be deve

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