STRUCTURE OF THE VERTEBRATES 249 



enucleated cell passing into the capillary. The functional life of 

 the cell is evidently ver}^ short. 



The erythrocytes are disc-shaped and bi-concave, and their 

 size varies in different groups of mammals. Microscopic examina- 

 tion of the blood can often determine the genus from which it 

 was taken, even in a dried specimen. The usual circular shape 

 of the mammalian erythrocytes has an exception in the camel 

 and its relatives. In these animals the red cells are elliptical in 

 shape, although they agree with all other mammals in lacking 

 nuclei. In pathological conditions any mammal may throw 

 nucleated cells into the blood stream but these are rarely, if 

 ever, numerous. 



