CLASS MAMMALIA 421 



cavity is, of course, filled with amniotic fluid. When the young animal 

 is born, the umbilical cord is either ruptured or is severed soon afterward. 

 The placenta, which is called the afterbirth and often forms a mass of 

 considerable size, is immediately passed out. Since in the birth of a 

 human infant the umbilical cord is not ruptured, it has to be cut, the 

 end attached to the infant being tied to prevent hemorrhage. 



The process described above is subject to a variety of modifications 

 in different groups of mammals. Sometimes the amnion is formed as a 

 circular fold of ectoderm and mesoderm somewhat as in birds, the 

 margins of the fold coming together to form the amniotic cavity. The 

 placenta takes a variety of shapes in different mammalian types. Birth 

 takes place in different groups with the young in various stages of develop- 

 ment. In such animals as cattle and horses the young at birth are 

 well-developed, have their eyes open, and are soon able to walk and run, 

 needing the care of the mother only at the time of feeding. The young 

 of various carnivores and rodents, however, are born naked, blind, and 

 helpless and have to be cared for during a considerable period of time. 

 The human child, while not blind at birth, is nevertheless quite helpless 

 and demands parental care longer than the young of any other animal. 



448. Economic Importance. — Mammals are economically important 

 for many reasons. Among them are animals which for ages have served 

 man for food, both their flesh and milk being used. Their hides have 

 furnished leather and fur for the manufacture of clothing and for a 

 variety of other purposes. Horses, asses, camels, cattle, and other 

 mammals have been used as beasts of burden and have assisted man in 

 his labor of cultivating the soil. Of all mammals perhaps the horse 

 has played the largest part in the development of human civilization. 

 Mammals have also been the pets and associates of man since early in 

 his history. INIany mammalian products have been important articles 

 of commerce, including musk, which is the product of certain glands 

 of ruminants; ivory, which is supplied mainly by the tusks of walruses 

 and elephants; oil, which has been secured from the fat of sperm whales; 

 and ambergris, a product of the intestinal canal of whales used as a 

 base in the manufacture of fine perfumes. Formerly whalebone was 

 an important article of commerce but its value has diminished in recent 

 years. 



Not only are many mammals valuable to man, but the group also 

 includes some which are decidedly injurious. Among these are rodents, 

 which destroy crops in the fields or commit ravages about houses and 

 outbuildings. The rat is injurious not only for this reason but because 

 it is also a carrier and distributor of the germs of disease. In some 

 countries, particularly tropical ones, wild mammals are a menace to the 

 lives of people, and everywhere carnivorous mammals are a constant 

 threat to the safety of cattle and other domesticated animals. 



