BLOOD SYSTEM. 



355 



another in shape and structure, though varying in .size. 

 In shape they are all elliptic; in structure they are nucle- 

 ated; in size they vary from nearly the size found in 

 mammals in certain fishes to very many times that size 

 in some amphibians. The largest are found in some 



Mam. 



Bird 



»/, 



7,sA 



FIG. 237.— Blood of different classes of vertebrates, showing comparative 

 sizes. All are equally magnified : m, man ; el, elephant ; ms, mouse ; 

 m, musk deer ; lib, humming-bird ; p, pigeon ; est, ostrich ; sn, snake ; 

 t, toad ; pr, proteus ; p, perch ; pk, pike ; sh, shark. 



tailed amphibians, such as the proteus, in which they are 

 ten times the diameter or one hundred times the surface 

 area of those of man (Fig. 237). 



All these classes of vertebrates are egg-laying. 

 Therefore blood containing this style of red corpuscles 

 may be called oviparous vertebrate blood. 



All vertebrate blood (unless we except that of the 



