232 



THE PRINCIPLES OF AGRICULTURE 



work. A cell which has to do only with nutrition is special- 

 ized ; one which has to do with nutrition, sensation, locomotion, 

 and reproduction, is generalized. A cell may be said to be 



Fig. 84. Amoeba, showing large, round nucleus 

 near the top, enclosing a uucleolus, many 

 granules, protruding arms of protoplasm, Fig. 85. 



and white space round which the proto- Muscle cell, 



plasm has flowed. Magnified 200 diameters. Magnified. 



still further specialized when it carries on some particular or 

 special part of nutrition. 



363a. A secretion is a material derived from the blood and 

 poured out into the body. When this material is of no further 

 use, it is eliminated, or removed from the body, and is known as 

 an excretion. The saliva, eye-water, bile, gastric juice, are ex- 

 amples of secretions. 



363ft. Glands are secreting organs. Thus the salivary glands 

 secrete or make the saliva or spittle, from the blood. The 

 liver is a gigantic gland, secreting bile and other materials. 



364o. Glycogen is very like starch. In fact, it has the same 

 chemical composition, C 8 HioO 5 . It is rapidly changed into grape 

 sugar or glucose by the action of saliva and other juices, and 

 it then becomes available for the building of tissue or keeping 

 up the bodily heat, 



