At a later period of uterine development, when the fœtus, measured in a straight line from the mouth to the root of the tail, is ten lines in length, the urachus expands into a small allantois, fig. 569, d, of a flattened pyriform figure, and finely Uterine fœtus, membranes and appendages, Macropus major. (The fœtus is magnißed twice the natural size.) wrinkled external surface. This bag insinuates itself between the amnios and chorion, carrying along with it two small hypogastric arteries and a vein, but not establishing by their means an organised 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 cord, and has no connection at any part of its circumference with the adjoining membrane. Its office, as in the Batrachia, is apparently limited to that of a receptacle of urine. The vitellicle orumbilical vesicle' presented the same large proportionate size and vascular structure as in the first described foetus. The chorion which enveloped this foetus 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 organised by the extension of the omphalo-mesenteric vessels upon it from the adherent vitellicle. The digits of the hind legs were distinctly formed in this embryo. In some smaller kinds of Kangaroo an ovum from each ovary may be impregnated, and two embryos be simultaneously developed.' Rengger gives the following account of the generation of a species of Opossum (Didelphis Azare):- The foetuses are developed in the cornua uteri, and not in the lateral canals. Some days after impregnation they have the form of small round gelatinous corpuscles, which do not appear, even when examined with a lens, to have any communication with the mother, but a red line indicates the first commencement of development. Towards the end of gestation, when the foetuses have attained the length of six lines, they are seen to be enveloped in a membrane and provided with an umbilical cord, which is united to the uterus (chorion?) by the medium of many filaments. The head, the four extremities, and tail are recognisable with the naked eye, but those fœtuses which are nearest the Fallopian tubes are generally least advanced. In gestation they make the circuit of the lateral canals, in which they are found to be deprived of their fœtal envelopes, and to have no communication with the parent by means of the umbilical cord; whilst one foetus was found in this situation, two others were still in the body of the uterus ' (vaginal cul-de-sac ?), from which the umbilical cords were not yet detached. At this period a slight enlargement of the Two have not been found in the same uterus. Mr. Collie, Surgeon, R.N., states, I have just now procured gravid uteri (of the Macropus Brunii) in which two fœtuses seem to be arrived at, or very near to, the termination of the period of gestation. One of them, which was about one-half larger than the body of the common wasp, has protruded through an opening inadvertently made in the uterus, and is distinctly seen through its transparent membranes and the liquor amnii.'- Zoological Journal,' vol. V. p. 240. uterus and lateral canals was the only change perceptible in them."1 2 As accomplished Naturalists continued to believe and affirm that the young of the Marsupialia quitted the womb and were received into the pouch in the condition of a gelatinous ovum comparable to a Medusa,' I deemed it requisite to anatomise the rare instance of the uterine foetus of the Kangaroo, in order to demonstrate the conditions of the respiratory, circulating, digestive, and renal systems. From the cæcum, 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 large equal and symmetrically disposed lobes. The vena porta was formed by the union of the vitelline with the mesenteric veins. 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. 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-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 From the Analysis of Rengger's Säugethiere von Paraguay' in the Bulletin des Sciences Nat. tom. xxi. p. 469. 2 CCLXIV". p. 342. 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." The nervous system alone is embryonic, fig. 585. § 401. Development of Lissencephala.-In Lyencephala, of which the uteri undergo least change of dimensions during the characteristic brief gestation, those tubes have the definitely organised and persistent lining membrane described, pp. 679, 685. 570 In the placental Mammals of which the uteri undergo wholly or locally great and rapid changes of size and capacity the lining of the incubatory part is less differentiated. In some it is but remotely allied to the class of membranes called 'mucous': canals so lined are habitually traversed by the matters they have to convey. The transmitting function of the womb is seldom exercised in the course of life, with long intervals of rest: its lining has a higher, organising, office: it differs from mucous membrane in the absence of submucous areolar tissue' (p. 439, fig. 361, c, d), or any such medium of connection with, or reception of, vessels from the muscular coat: it is a pulpy substance, in which corpuscles or nuclei abundantly, and fibre-cells more sparingly, formify: it receives directly from the fleshy substance of the womb its vascular supply; and is perforated by the minute canals, sparingly exuding fluid, termed uterine or utricular glands.' A ciliate epithelium may be distinguished on the free surface in most Mammals; non-ciliate cell-deposit occupies more or less of the ' utriculi.' These, in fig. 570, are indicated by the pale tortuous Section of lining substance, human uterus; nat. size. 1 LXXV. p. 337 (1834). Such is the difficulty of giving up a strange or telling' statement, which has once gained currency, that we read:- Les petits ne se développent pas comme d'ordinaire dans la poche utérine, mais sont promptement expulsés au dehors, et naissent dans un état d'imperfection telle qu'on ne peut les comparer qu'à des embryons à peine ébauchés. Ce sont des petits corps gélatineux, informes et incapables de mouvement, dont les divers organes ne sont pas encore distincts, et dont l'existence serait impossible si la nature n'avait assuré leur conservation par des moyens particuliers.' CCLXV". Even M. Pouchet, usually so conscientiously accurate, writes:-Le produit de la génération qui, en arrivant là (the pouch), n'est qu'un simple ovule encore baigné de fluides albumineux, se trouve posé sur les tétines.' (ccxcr". p. 262, 1841). My observations on the Kangaroo were confirmed by those of Meigs on the Opossum (CCLXXVIII". 1847), lines a b; the dark fine lines represent injected capillaries, continued directly from the fibrous walls of the uterus, a c. Such pulpy vascular substance, compared by JOHN HUNTER to 'coagulable lymph," is rapidly formed, readily shed, speedily renewed: it grows with the needs of the growing embryo or fœtus, as the medium for bringing into requisite relation with the circulating system of such the mother's blood. In the uterus with a non-caducous and well-organised lining membrane the chorion or outer coat of the ovum continues smooth and unvascular, at least, until the fœtus and appendages have advanced to the degree shown in fig. 569. But in the deciduate type of lining substance there is a reciprocal preparation 571 Uterine ova, Rabbit. CCCVIII. of the chorion for intimate In the very small pro- In the Rabbit, before the ovum enters the uterus, it has received from the oviduct additional layers, ib. A, B, which, either in combination with, or substitution for, the hyalinion, become the medium of applying the capillaries of the foetus to those of the mother continued into the decidua. When the foetal appendages have attained, in this Rodent, the stage they exhibit in the Marsupial, fig. 569-viz. the formation of the amniotic bag, fig. 571, a, the inclosure of the unconverted germ-mass, or yolk, by a vascular vitellicle, b, and the out-budding of a small allantois, c-the embryo is by no means so far advanced. The ventral parietes of the thoracic-abdominal cavity are not formed, the bilocular heart is In the indication of the various definable parts of this deciduous substance a vocabulary of terms has been created; e. g. non-placental uterine mucous membrane,' 'uteroplacental mucous membrane,' persistent,' or non-deciduous serotina,' 'deciduous serotina,'' fimbriae of decidua serotina,' 'membrana decidua,' 'decidua reflexa,' rags,' 'tags,' &c. |