This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* Owing to an error on the part of the draughtsman, who has neglected to reverse the drawing, the left uterus in the above figure is represented on the right side, and vice versa.
(2427). From the above considerations it is therefore probable that the young Ornithorhynchi are produced alive; yet still the reader will perceive, by the closeness of the reasoning brought to bear upon the subject, how nearly the oviparous and mammiferous modes of generation are approximated by the interposition of these connecting forms of Vertebrata.
(2428). But if from these arguments, derived from the anatomical construction of the female parts, it is allowable to conjecture that the Orni-thorhynchus is ovoviviparous (using that term in a strictly philosophical sense), the difficulties of the case are by no means removed; and granting that the contents of the ovum are barely sufficient to nourish the embryo during the very earliest stages of its development, we have yet to learn how the foetus is matured after the exhaustion of this supply. There is no reason whatever to suppose that a placenta exists at any period of uterine gestation; neither is there a marsupial pouch in which the prematurely-born young can be carried about and supplied with milk; so that, whether the young Monotreme be developed in the uterus or out of the uterus, we are equally at a loss to understand how its nutrition is provided for.
(2429). In this state of uncertainty, the anatomy of the young Orni-thorynchus, examined at as early a period as possible, becomes a subject of extreme interest; and fortunately Professor Owen has been enabled to add observations upon this subject to his other valuable researches relative to the generation of these creatures *. The annexed figure (fig. 415) is a portrait of one of the specimens dissected; and from every appearance it could not have been more than a few days old - that is, supposing it to have been born at an advanced period of its development. It was as yet blind; and the situation of the eyes was only indicated by the convergence of a few wrinkles to one point; but when these were put upon the stretch, the integument was found entire, and completely shrouding or covering the eyeball anteriorly: its skeleton, moreover, was quite in a cartilaginous condition; and it was obviously in every respect helpless, and still dependent upon its mother for sustenance.
Fig. 415. Young Ornithorhynchus.
(2430). The stomach was found filled with milk - a sufficient proof that at that period, at least, it was nourished by the lacteal secretion; but with regard to its previous foetal condition, the difficulties that have been above alluded to remained in their full force. No trace of an umbilical cicatrix was visible upon the ventral surface of the body, even when examined with a lens, - a sure proof that no placenta had existed. The ilium was carefully examined, but there was no appearance of the pedicle of the vitelline vesicle; nevertheless the other vestiges of foetal organization were more obvious than in the ordinary marsupial or ovo-viviparous Mammalia. The umbilical vein was seen extending from a linear cicatrix of the peritoneum, opposite the middle of the abdomen, along the anterior margin of the suspensory ligament to the liver. It was reduced to a mere filamentary tube filled with coagulum. From the same cicatrix the remains of the umbilical arteries extended downwards, and near the urinary bladder were contained within a duplicature of peritoneum, having between them a small flat oval vesicle, the remains of an allantois, which was attached by a contracted pedicle to the fundus of the bladder; but still, as both the embryo of the Bird and that of the ovoviviparous Reptile have an allantois and umbilical vessels developed, no certain inference can be drawn from the above appearances as to the oviparous or viviparous nature of the generation of the Ornithorhynchus.
(2431). Such is the present state of our knowledge relative to the first type of Mammiferous generation, viz. that met with among the Monotremata. In the second, or Marsupial type, the phenomena, although equally strange, are better understood; and to these we must now beg the attention of the student.
(2432). The Marsupialia, from the variety of their forms and extensive distribution, constitute a most important section of Mammiferous quadrupeds, distinguished by the peculiarities that occur in the organization of their generative apparatus and by the singular mode in which they produce and suckle their young. Animals of this kind are only met with in the American and Australian regions of our globe; and so widely do they differ, as far as their reproduction is concerned, from all the Mammiferous inhabitants of the Old World, that they might even be regarded as forming quite a distinct and separate group in the animal creation, serving to accomplish another step in that grand transition by which the physiologist is conducted from the oviparous to the placental Vertebrata.
* Owen on the Young of the Ornithorhynchus paradoxus, Trans. Zool. Soc. vol. i.
(2433). The Marsupialia are, strictly speaking, ovoviviparous; that is to say, the uterine ovum never forms any vascular connexion with the maternal system, but after a very brief intra-uterine gestation the embryo is expelled in a very rudimentary and imperfect condition, even its extremities being as yet but partially developed; and in this helpless state the foetus is conveyed from the uterus into a pouch or marsu-pium, formed by the integument of the abdomen, there to be nourished by milk sucked from the mammary glands, until it arrives at such a state of maturity as enables it to assume an independent existence.
(2434). We may naturally expect, therefore, that, with habits so remarkable, the structure of the generative apparatus, both in the male and female Marsupial, will offer important peculiarities; and these accordingly first present themselves for description.