818 MAMMALIA. 



(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 re- 

 markably 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 com- 

 position, 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 Yertebrata lay eggs, and their young are per- 

 fected 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 in- 

 dependent 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 Lemur and the Mole form remarkable exceptions ; for in these creatures the 

 female urethra traverses the clitoris precisely as in the other sex. 



