i6o 



COMPARATIVE ANATOMY 



In human blood the red cells outnumber the white in the ratio of five or 

 six hundred to one. 



Erythrocytes (Figs. 122 and 123) are relatively small and usually have 

 the form of flat discs with elliptical outlines. These blood cells are heavily 

 loaded with hemoglobin, a very complex protein substance containing iron 

 and having a strong affinity for oxygen which the cells pick up at the 

 respiratory surfaces of the animal. The red cells are therefore the oxygen- 

 carriers. Their color is due to the hemoglobin. The mature erythrocytes 

 of all vertebrates except mammals are nucleated, although the nucleus 



Fig. 122. — Coagulated blood. Biconcave red corpuscles arranged in "rouleaux"; 

 filaments of fibrin radiating from minute blood plates. (From Bremer, Text-book of 

 Histology; after Da Costa.) 



seems to be in a more or less degenerate condition. But in adult 

 mammals the red cells in course of their differentiation lose their nuclei, 

 thereby acquiring the form of concavo-convex discs (Fig. 122). 



Erythrocytes are produced in mesenchymal tissue in the liver and in 

 the spleen of embryos, but in the adult their chief source is probably the 

 red bone-marrow. They serve as oxygen-carriers for a limited time and 



Fig. 123. — Cells from smear preparation of normal human blood, Wright's stain. 

 In the center: adult red blood corpuscles, blood platelets and a polymorphonuclear 

 neutrophile. At left above: two polymorphonuclear basophiles and two polymorphonu- 

 clear eosinophiles. At right above: three large and four small lymphocytes. At left 

 below: polymorphonuclear neutrophiles; two of these cells, the uppermost and lower- 

 most of the group, are young, with merely crooked nuclei; the mature cells have multi- 

 lobed nuclei. At right below: six monocytes; in the younger cells the nuclei tend to be 

 rounded, in the adult cells they are horseshoe-shaped, indented or lobed. (From 

 Bremer, Text-book of Histology.) 



