CLASS MAMMALIA 447 



and the placenta, the two being connected only by a body stalk, which 

 represents the original connection between the trophoderm and the 

 inner cell mass. In some cases the allantois grows into this body stalk 

 and assists in the formation of the placenta. Such a placenta, accord- 

 ingly, is termed an allantoic placenta. If the allantois remains rudimen- 

 tary and does not enter the placenta, the latter is called a chorionic 

 placenta; in this case the mesodermal layer of the chorion becomes 

 much thickened and very vascular. Whether the placenta is allantoic 

 or chorionic the blood vessels of the mother come into intimate contact 

 \^^th those of the embryo, though the two sets of vessels never actually 

 communicate. As a result of this condition a free interchange of sub- 

 stances in solution occurs. From the maternal blood oxygen and food 

 are passed into the fetal circulation, while carbon dioxide and waste 

 substances pass in the opposite direction. In the case of the human 

 embryo and of other forms the amniotic cavity becomes very large, the 

 extra-embryonic coelom becoming correspondingly reduced, or even 

 eliminated, and the embryo comes to lie suspended in the amniotic cavity 

 by the body stalk, which is called the umbilical cord. This amniotic 

 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. 



450. 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 



