(2409). Widely different, however, is the arrangement of the male genito -urinary system in the class we are now considering. The cloacal cavity is no longer met with, the terminations of the rectum and of the sexual ducts being now remotely separated; the penis is traversed by a complete urethral canal, through which the seminal fluid is forcibly ejaculated; and moreover, subsidiary glands not met with in any of the preceding classes add their secretions to that of the testes, and thus facilitate the intromission of the fecundating fluid. A urinary bladder is now superadded to the renal apparatus, wherein the urine is permitted to accumulate in considerable quantities, prior to its expulsion through the urethra - the excretory duct common to both the urinary and generative organs.

(2410). Not less remarkable are the corresponding changes observable in the disposition of the female reproductive organs. The Mammifers are appointed to bring forth living young; a uterine receptacle is therefore necessarily provided for the reception of the foetus, and mammary glands are given to support the tender offspring during the earlier portion of its existence. But the history of these organs cannot be laid before the reader at a glance, and we must therefore patiently trace out their development step by step, and gradually ascend from the Oviparous type up to the most complete forms of the genito-urinary system.

(2411). Commencing with the urinary apparatus, the first parts that offer themselves to our notice are the kidneys, the ureters, and the bladder, in describing which the same remarks will be found applicable to both sexes.

(2412). The kidneys in all the Mammiferous orders occupy a similar position, being situated in the loins, on each side of the aorta, from whence they receive a copious supply of arterial blood by the renal arteries, which, after having supplied the urinary secretion, is returned to the circulation by the emulgent veins that empty themselves into the inferior cava.

(2413). As relates to their intimate structure, the kidneys of all quadrupeds are essentially similar to those of our own species, each of these organs being composed of uriniferous tubules of extreme tenuity that terminate in central papillae from which the urine flows. These tubules, as they advance into the medullary substance of the kidney, bifurcate again and again, until they arrive at the cortical or external portion, where they spread out on all sides, and, becoming exceedingly flexuous, are inextricably intervolved among each other, so that the entire cortex is composed of their gyrations. At last all the uriniferous vessels terminate in blind extremities, and, according to Muller*, have no immediate communication with the vascular system.

* De Glandularum Structura, p. 102.

(2414). In form the kidneys of Mammals more or less resemble the human; but there is one important circumstance observable in many tribes, which is well calculated to show that these organs, even when they appear most simple, are in reality formed by the coalescence of several distinct glands. In the human foetus the kidneys present a lobulated appearance; that is to say, they are evidently composed of numerous divisions, each having the same structure; but in the adult the lines of demarcation between these lobes become entirely obliterated. In many genera, however, this division into lobes remains permanent during the whole lifetime of the creature: such, for example, is remarkably the case in amphibious Carnivora, as the Otter and the Seal tribes, and still more strikingly in the Cetaceans, where the kidneys are not inaptly comparable to large bunches of grapes. But whatever the form of the organ, or the number of lobules entering into its composition, the urine secreted by each kidney is received into a common excretory duct called the ureter, and is thus conveyed into the bladder prepared for its reception.

(2415). The urinary bladder exists in all the Mammalia, and receives the ureters by valvular orifices in precisely the same manner as in the human subject. In the male its excretory duct, the urethra, is common to the urinary and generative systems, and terminates at the extremity of the penis; but in the female the urethral canal is of much simpler structure, opening by a distinct orifice into the vulva *.

(2416). We have preferred laying before the reader the above general view of the urinary system of Mammalia to noticing in detail those varieties that occur in the disposition of the bladder and urethra of some of the lower tribes, in conformity with the different types of organization presented by their sexual organs; these, however, must not be lost sight of in following out the development of the reproductive apparatus, from the oviparous races to the most perfect and highly-gifted members of the animal creation. It is to this important subject that we must now invite the attention of the reader.

(2417). The oviparous Vertebrata lay eggs, and their young are perfected without further nourishment derived from the maternal system than is contained within the egg itself. In our own species, and throughout all the races of Mammalia found on the European continent, the females produce their young alive and fully formed, capable of independent existence, but, nevertheless, nourished for a considerable period by milk derived from the breast of the mother. The distinction, therefore, between an oviparous and a viviparous creature would appear to be sufficiently broad, and the physiological relations between them as remote as possible.

(2418). The student, however, who has followed us thus far through the long series of living beings that have successively presented themselves to our notice must naturally expect that, between animals so dissimilar in their economy as the Bird and the Mammal, intermediate types of organization must occur, and that the transition from one to the other is here, as elsewhere, gradually accomplished.