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.
* The Lemur and the Mole form remarkable exceptions; for in these creatures the female urethra traverses the clitoris precisely as in the other sex.
(2419). In this respect his expectations will be by no means disappointed. The Ornithorhynchus paradoxus and the Echidna, animals met with only in the continent of New Holland, are most obviously connecting links between these two grand classes; and, therefore, it is with the history of these strange animals that we must commence our examination of the Mammiferous generative system.
(2420). The Ornithorhynchus paradoxus well deserves the specific epithet applied to it by zoologists. It has, indeed, the form of a quadruped, and its body is covered with hair, and not with feathers; but its mouth is the beak of a duck; and upon its hind feet, which are broadly webbed, the male carries a spur not unlike that of a barn-door fowl. Having the beak of a bird, how is the creature to suck? Nevertheless the females have mammary glands well developed, but destitute of prominent nipples; so that the mode in which the young animal obtains the milk provided for it is even yet a puzzling question. Does the Ornithorhynchus lay eggs? or produce living young ones? This is a query that has not been satisfactorily answered; and its generative apparatus is so nearly related to that of an oviparous animal that even anatomy throws but little light upon the subject.
(2421). Both in the male and female there is, in fact, but one vent, that leads to a cloacal chamber resembling that of a bird; and the entire organization of the sexual organs is rather that of an egg-laying than of a viviparous creature, as will be evident from the following details respecting them.
(2422). The penis of the male Ornithorhynchus is perforated by a urethral canal, through which the semen passes, but not the urine; its extremity, moreover, is terminated by two tubercles, giving it almost a bifid appearance. This penis, when in a relaxed state, is lodged in a little pouch in the floor of the cloaca, from which it projects when erected.
Fig. 412. Male generative organs of Ornithorhynchus paradoxus.
(2423). The cloacal cavity, as in birds, gives passage to the faeces and to the urine. The testes (fig. 412, a) and the vasa deferentia (b) resemble those of an oviparous animal; but, on the other hand, there is a complete urinary bladder (c), and moreover a pair of auxiliary (Cowper's) glands (d d), organs never met with except in the Mammiferous class.
(2424). The anatomy of the female organs is not less singular. The ovaria (fig. 414, a a) are large and racemose, like those of a bird; while the two oviducts or uteri (fig. 413, a a), as the reader may choose to call them, open into the cloaca by two distinct orifices (c c), situated on each side of the urethra, derived from the bladder (6).
(2425). It is to Professor Owen that science is indebted for all that is known relative to the anatomy of the female Ornithorhynchus when in a gravid state; and his researches upon this subject appear to establish the following interesting particulars: - First, that the ovaria, notwithstanding their racemose appearance, exhibit all the essential characters of the Mammiferous type of structure; and corpora lutea were formed where the reproductive germs had escaped from them. Secondly, that the eggs contained in the uterine cavities (fig. 414, c, e) had no connexion whatever with the walls of the uterus. Thirdly, that each ovum exhibited the usual parts of an egg, viz. the cortical membrane, the albumen, and the yelk; and that upon the latter a memhrana vitelli and the blastoderm or germinative membrane were plainly perceptible. Fourthly, that the uterine walls assume an increased thickness when in an impregnated state, but that not the slightest trace of a decidual or adventitious membrane is apparent in the cavity of the womb. From all these circumstances, the distinguished author of the paper referred to* was led to adopt the subjoined train of reasoning as to the probability of the Ornithorhynchus being a viviparous mammal. The form, the structure, and the detached condition of the ova, observes Professor Owen, may still be regarded as compatible with, and perhaps favourable to, the opinion that they are excluded as such, and that the embryo is developed out of the parent's body. But the following objections present themselves to this conclusion: - The only part of the efferent tube of the generative apparatus which can be compared in structure or relative position with the shell-secreting uterus of the Fowl is the dilated terminal cavity in which, in all the specimens examined, the ova were situated: and upon the oviparous theory it must be supposed either that the parietes of this cavity, after having secreted the requisite quantity of soft material, suddenly assume a new function, and complete the ovum by providing it with the calcareous covering necessary to enable it to sustain the superincumbent weight of the mother during incubation; or that this is effected by a rapid deposition from the cuticular surface of the external passages; or lastly, according to a more recent but still more improbable supposition, by a calcareous secretion of the abdominal glands poured out upon the ovum after its exclusion.
Fig. 413. Generative organs of female Ornithorhynchus.
* "On the Ova of the Ornithorhynchus paradoxus," by Richard Owen, Esq., Phil. Trans, pt. ii. for 1834, p. 563.
Fig. 414. Ovaria of Ornithorhynchus paradoxus.
(2426). But granting that the egg is provided in any of these ways with the necessary external covering, yet, from the evidence afforded by the specimens examined, the ovum is deficient in those parts of its organization which appear to be essential to successful incubation, viz. a voluminous yelk to support the germinal membrane, and the mechanism for bringing the cicatricula into contiguity with the body of the parent. Add to this, that such a mode of development of the foetus requires that all the necessary nutritive material be accumulated in the ovum prior to its exclusion. Now the bony pelvis of the Bird is expressly modified to allow of the escape of an egg, both large from the quantity of its contents, and unyielding from its necessary defensive covering: but, whatever affinities of structure may exist in other parts of the Ornitho-rhynchus, it is most important to the question of its generation to bear in mind that it manifests no resemblance to the Bird in the disposition of its pubic bones.