ANIMAL ORGANISMS 431 



(Sees. Ill, 115, 116, 137, 175). One is the differentiation which exists 

 within the individual cell; a second is that of cells in the many-celled 

 organism, which results in the formation of tissues; a third is that of the 

 individuals in a colonial organism, as in the Portuguese man-of-war. 

 Division of labor may also appear in a society, as in social bees and 

 ants, accompanied by polymorphism. Differentiation within individuals 

 exists throughout the animal kingdom, but it is carried to the highest 

 degree in the highest organisms. 



459. Integration. — Parallel with differentiation occurs a process 

 known as integration. Differentiation appears as the result of differences 

 which develop in the parts of an organism. Integration is brought about 

 by such an intimate coordination between these various parts as to 

 result in unity of action and tends to increase the efficiency of the organ- 

 ism as a whole. Integration is shown in the association of tissues to 

 form organs and of organs to form systems and in the coordination of all 

 parts in the activities of the entire organism. Just as in the highest 

 animals differentiation reaches its highest expression, so in the same 

 animals integration is developed to the greatest degree. Two factors 

 concerned in coordination are centralization in the nervous system and 

 chemical control in other systems of the body. 



460. Centralization. — Centralization, which involves the development 

 of a central nervous system, has been traced through the different phyla 

 and has been seen to culminate in the highly centralized nervous system 

 of the vertebrates. In the higher organisms has been added cephalization, 

 which is the setting apart of a head region containing the brain and many 

 of the organs of special sense (Chap. XXXI and Sees. 264, 273, 280, 331, 

 335, 350). 



461. Chemical Control. — The chemical control of one part of the 

 body over another is exercised through the production of substances 

 W'hich may be included under the general term of hormones. A hormone 

 may be defined as a substance of chemical nature produced in one part 

 of the body which, when carried to another part, serves to stimulate it to 

 activity. In this broad sense the term covers all internal secretions. 

 The glands which produce these are often called the endocrine glands. 

 The discovery of hormones is comparatively recent and our knowledge 

 of them is far from complete. Nevertheless, certain facts may be 

 definitely stated. 



An example of a hormone is presented by the gastric secretin. The 

 stomach is called into action by the stimulation of the vagus nerve. 

 This effect soon wears off but the cells are caused to continue functioning 

 by the gastric secretin which is formed by the cells themselves and passed 

 into the blood. Similar substances are produced by the pancreas and 

 liver. The ovary of a pregnant mammal produces a hormone which, 

 carried through the blood to the mammary gland, stimulates the latter 



