42 GENERAL BIOLOGY 



of this intercellular substance the connective tissue 

 is described as white fibrous tissue, yellow elastic 

 tissue, cartilage, bone, etc. Strikingly dissimilar 

 as these types of tissue are, they not only are ex- 

 amples of the same sort of protoplasmic activity, 

 but all of them in development have been shown to 

 be derived from an original, much more generalized 

 type of primitive connective tissue cell. 



In the other phase of secretory activity mentioned, 

 the production of secretions which are freely trans- 

 ported to other parts of the organism, we have a 

 very easily apprehended example of the division of 

 labor between the parts. 



Specialization in Digestion. Amoeba, when it has 

 ingested a particle of food-substance, digests it by 

 dissolving it so that it can be assimilated into the 

 protoplasm. It does so by means of minute quan- 

 tities of a substance which it secretes that acts 

 chemically upon the food. (See Chapter III.) This 

 process of digestion, like that of ingestion, may take 

 place in all parts of the cell-body, i.e. it is not localized. 

 In the complex structures composing the body of a 

 higher animal it is obvious that it would be impossible 

 for food to come in contact with every part of the 

 body so that each cell could attend to its own 

 function of digestion. Accordingly, we find that 

 the principle of the physiological division of labor 

 comes into play in very simple and otherwise quite 

 generalized forms (such as Hydra}. The secretion 

 of digestive fluids and the function of alimentation 



