178 BIOLOGY AND HUMAN LIFE 



distributed by the blood and have an important influence upon the growth 

 of the animal. Another of these substances that affects growth is absorbed 

 by the blood from a little body lying at the base of the brain, the pituitary. 

 These various substances, or ferments, are sometimes called internal 

 secretions, because they are absorbed by the blood directly from the 

 tissues of these special organs, instead of coming out through ducts, or 

 tubes, as is the case with the more familiar glands. These internal secre- 

 tions are also called hormones. 



146. The red corpuscles. These cells are found in all verte- 

 brates. They contain hemoglobin, which combines chemically 

 with either oxygen or carbon dioxid, according to which is more 

 abundant. This makes the red corpuscle a gas carrier floating 

 in the plasma (see Fig. 85). 



Red corpuscles, like the white ones, are really unattached cells. They 

 originate by cell division of special cells in the marrow of bones. At first 

 they have a nucleus, but this soon disappears.^ The older corpuscles go 

 to pieces, and their hemoglobin is taken up by the liver and converted 

 into part of the bile (see section 113). 



147. The white corpuscles. We owe much of our understand- 

 ing of the white corpuscles to the great Russian biologist, 

 Elie Metchnikoff, who was director of the Pasteur Institute 

 in Paris. Like Ameba (see page 59), these cells consist of 

 naked protoplasm and have no fixed shape. Whereas the one 

 cell of the ameba carries on all the functions of a living body, 

 the various cells of a many-celled animal like an ant or a baby 

 have specialized functions as well as specialized structures. 

 Now the white corpuscles are in many ways the least specialized 

 cells in the body. They have the general qualities of protoplasm 

 in the greatest degree. 



I. As eating cells (or phagoc5rtes, which means ''eating cells," 

 as they were called 'by Metchnikoff ) they are capable of flow- 

 ing about or engulfing foreign particles with which they may 

 come in contact. They may swallow and digest dead particles 



1 Among vertebrates other than mammals the corpuscle retains its nucleus. 



