DEVELOPMENT AND LACTATION OF MAMMALIA. 93 



bare, and whose body, instead of resembling that of a Dog, a 

 Horse, or any ordinary Mammal, presents the form belonging to 

 Fishes. Such are the Porpoise (Fig. 45), the Whale, and other 

 Mammals of the order Cetacea, which are commonly, but errone- 

 ously, reckoned as Fishes. ( 25.) 



73. Development and Lactation. That which is most dis- 

 tinctive of the Mammalia, is their mode of development and of 

 nourishment during the earliest period of life. These animals 

 are all born alive ; and in their previous embryonic condition, 

 their life is not supported by a separate store of nutritive matter, 

 as is the case amongst oviparous animals, but they derive their 

 nourishment direct from the blood of the mother ; and, after 

 birth, the young still live at her expense, as she suckles them for 

 a longer or shorter time. The milk, destined for this purpose, is 

 a white and opaque liquid, composed of water, holding in solu- 

 tion sugar, casein (which forms the curd), with certain salts ; 

 and holding in suspension some globules of butter. Its qualities 

 vary a little amongst different animals, and may be modified by 

 the food of which they make use. In general it leaves, by evapor- 

 ation, from ten to twelve per cent, of solid parts ; but its rich- 

 ness may vary much according to circumstances. This nutri- 

 tious liquid is secreted by particular glands, named mammary, 

 which exist in both sexes, but which in the females alone serve 

 for the support of the young. Analogous organs are not to be 

 met with in any other class of the Animal Kingdom ; and it is 

 from their existence amongst all the animals composing the group 

 whose history we are commencing, that Zoologists have given to 

 these beings the name of MAMMALIA. The number of the mam- 

 mary glands is in general nearly in proportion to that of the 

 young produced at each birth. There are often only two (amongst 

 the Monkeys, the Elephant, the Goat, and the Horse, for ex- 

 ample) ; but sometimes, also, their number is much more con- 

 siderable. Thus, the Cow, the Stag, the Lion, have four ; the 

 Cat, eight ; the Pig and the Rabbit, ten ; the Rat, ten or twelve ; 

 and the Agouti, twelve or fourteen. The position of these glands 

 varies also. Amongst the Monkeys and Bats they are placed 

 upon the thorax, as in man ; in the greater number of Carni- 



