HISTOLOGY OF HIGHER ANIMALS 



53 



Individually of a pale yellow colour, and so small as to be quite 

 invisible to the naked eye, they occur in such vast numbers as to 

 give the blood its characteristic scarlet or purple colour. It is 

 estimated that in a cubic millimetre of human blood there are 

 about five millions of these red corpuscles. 



In the frog the haematids are flattened oval cells about 0'02 mm. 

 in longer diameter, with a centrally placed nucleus (Fig. 15, b.). 

 In man they are a good deal smaller, and circular in out- 

 line, like biscuits, and, as in all the Mammalia, the nucleus 



FIG. 16. Epithelium from the Mesentery of a Frog, x 280. (From a 



photograph.) 



The underlying tissues are seen indistinctly through the transparent epithelial cells, 

 whose outlines only are visible. 



has entirely disappeared. They owe their red colour, and 

 their power to act as carriers of oxygen, to the presence in them 

 of a peculiar pigment known as haemoglobin, with which the 

 oxygen appears to enter into a state of loose chemical combination 

 from which it is easily liberated again when required by the tissues. 

 They may indeed be regarded as mere bags of hemoglobin, formed 

 from highly specialized cells which have lost all power of indepen- 

 dent existence. They cannot even multiply by division, but, as 

 the old ones are worn out, they are replaced by the formation 

 of new ones from less specialized cells in various parts of the 

 body. 



