A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 489 



breathing, which is certainly a desirable attribute for organisms with 

 a high rate of metabolism. Mammals can manipulate food in their 

 mouth and breathing need be interrupted only momentarily when the 

 food is swallowed, and in some species not even then (young of opos- 

 sum, p. 492). 



Mammals, like birds, have evolved an efficient system of internal 

 transport of materials between sites of intake, utilization and excretion. 

 Their heart is completely divided internally so there is no mixing of 

 venous and arterial blood. Venous blood coming from the body and 

 going to the lungs passes through the right atrium and right ventricle, 

 while arterial blood coming from the lungs and going to the body passes 

 through the left atrium and ventricle. Increased blood pressure also 

 makes for a more rapid and efficient circulation. 



Nitrogenous wastes from the breakdown of proteins and nucleic 

 acids must be eliminated without an excessive loss of body water. In 

 mammals, most of the nitrogenous wastes are eliminated in the form of 

 urea, which is more soluble and requires more water for its removal than 

 does the uric acid excreted by some reptiles and birds. Approximately 

 99 per cent of the water that starts down the kidney tubules is reab- 

 sorbed in special regions of the tubules, and the net loss of water is 

 minimal. The generally high metabolic rate of mammals results in the 

 formation of a large amount of wastes to be eliminated. An increase 

 in blood pressure, and hence in blood flow through the kidney, and an 

 increase in the number of kidney tubules have enabled mammals to in- 

 crease the rate of excretion. 



Care of the Young. The evolution by reptiles of the cleidoic egg 

 was a successful adjustment to terrestrial reproduction so long as verte- 

 brates were cold-blooded. However, embryos that are to develop into 

 homoiothermic adults must apparently have a warm, constant tempera- 

 ture to develop normally, so birds and mammals cannot lay eggs and 

 then ignore them. Birds lay cleidoic eggs, but incubate them by sitting 

 on them, and one group of primitive mammals, which includes the 

 duckbilled platypus of Australia, does the same. All other mammals are 

 viviparous. The eggs are retained within a specialized region of the 

 female reproductive tract, the uterus, and the young are born as minia- 

 ture adults. 



All of the extraembryonic membranes characteristic of reptiles are 

 present in viviparous mammals, but albuminous materials are not 

 ordinarily secreted about the egg. The allantois, or in a few species 

 the yolk sac, unites with the chorion, thereby carrying the fetal blood 

 vessels over to this outermost membrane. The vascularized chorion unites 

 in varying degrees with the uterine lining to form a placenta, in which 

 fetal and maternal blood streams come close together, though they 

 remain separated by some layers of tissue (Fig. 28.7). The embryo de- 

 rives its food and oxygen, and eliminates its carbon dioxide and nitrog- 

 enous wastes across these membranes. 



Care of the young does not stop at birth, for all female mammals 

 have specialized mammary glands, which secrete a nutrient milk on 

 which the young feed. In such primitive mammals as the platypus (Fig. 



