BLOOD AND BLOOD-FORMING ORGANS. 



ated elements ; or, second, the nonnucleated red cells are formed 

 from the nucleated by an absorption of the nucleus (or what appears 

 to be such to the eye of the observer, Arnold, 96) ; or, finally, the 

 nucleus is extruded from the original nucleated cell. According to 

 recent investigations (Howell) the third possibility represents the 

 change as it actually takes place. 



In all vertebrate animals except mammalia, the red blood- 

 corpuscles are nucleated. They are elliptic discs with a biconvex 

 center corresponding to the position of the nucleus. The blood- 

 cells of the amphibia (frog) are well adapted for study on account 

 of their size. They are long and, as a rule, contain an elongated 

 nucleus with a coarse, dense chromatin framework, which gives 

 them an almost homogeneous appearance. The cell-body may be 

 divided, as in mammalia, into stroma and hemoglobin. When sub- 

 jected to certain reagents, the contour of the cells appears double 

 and sharply defined. This condition is, however, no proof of the 

 existence of a membrane. The blood-cells of birds, reptiles and 

 fishes are similarly constructed. 



The diameter of the erythrocytes varies greatly in different ver- 

 tebrate animals, but is constant in each species. The red blood-cells 

 of man measure on the average 7.5 y. (7.2 p. to 7.8 //), in their long 

 diameter, and 1.6 fj. to 1.9/1 in their short diameter. We append 

 a table of their number in a cubic millimeter and size in man and 

 certain animals as compiled by Rollett (71, II) and M. Bethe : 



