CHAPTER VI 

 ORGANS AND ORGAN SYSTEMS 



i. GENERAL 



IN Hydra fusca, we have studied an animal composed only of 

 tissues, and a form representing the gastrula stage in develop- 

 ment of all higher animals. We have seen, moreover, that some 

 cells of Hydra, especially those of the ectoderm, are differen- 

 tiated for special functions, particularly those for protection and 

 nervous response. Such cells, however, are isolated and not 

 combined in compact tissues or special organs. In higher ani- 

 mals, we find similar cells derived from the ectoderm of the gas- 

 trula developing into complex organs of the skin and into still 

 more complicated organs of special sense, and finally into mar- 

 velously intricate organs like the human brain, spinal cord, and 

 sympathetic nervous system. In these higher animals also, cells 

 from the endoderm of the gastrula form all of the organs of 

 the usual animal metabolism. All of the usual mesodermal 

 structures, like bone, cartilage, muscles, connective-tissues, 

 mesenteries, blood, etc., are derived from cells originally set 

 apart from the vegetative cells of the gastrula. 



An organ differentiated for the special performance of some 

 vital function or part thereof never consists solely of the one 

 tissue mainly responsible for that function. The stomach, for 

 example, in which the first stages of protein digestion occur, does 

 not consist merely of secreting epithelial cells from the endo- 

 derm, but is a complicated aggregate of connective tissue, mus- 

 cle tissue and blood vessels from the mesoderm, and nerve tis- 

 sue from the ectoderm, giving support, contractility and food to 

 the functioning epithelial cells. The stomach, furthermore, per- 

 forms only a part of the function of food digestion; other organs 



130 



