CHAPTER XX 



THE DUCTLESS GLANDS 

 INTERNAL SECRETIONS 



IN unicellular organisms, as in the rest of the living world, activity 

 consists in adaptation to external conditions. The changes in the 

 environment which determine the reactions of these organisms may 

 occur at their surface or at some distance. Among the stimuli which, 

 acting from a distance, evoke the reaction of unicellular organisms, 

 probably the most important are those accompanied by chemical 

 changes. The interrelation of micro-organisms with one another is 

 determined almost entirely by such chemical stimuli. Thus the 

 antherozoids of ferns are attracted to the ovule in consequence of the 

 production in the tissue surrounding the latter of substances such as 

 malic acid. Among micro-organisms we find some which leave places 

 rich in oxygen, while others move towards any spot in their environ- 

 ment where oxygen is most plentiful. These reactions of unicellular 

 organisms to chemical stimuli are classed together under the term 

 chemiotaxis. When the cells are united together to form cell 

 colonies, or when, as in the metazoa, the multicellular aggregate is 

 formed by the failure of the products of division of the ovum to 

 separate one from Another, the interrelations between the different 

 cells forming the organism are still largely determined by chemical 

 stimuli. In fact, in the lowest metazoa, such as the sponge, we 

 know of no other means of correlating the reactions of different parts 

 of the cell aggregate. If a foreign substance be introduced into the 

 living tissue of a sponge it becomes speedily surrounded with a collec- 

 tion of wandering phagocytic cells, called from the surrounding parts 

 by the diffusion of chemical substances from the seat of the lesion 

 into the fluids of the body. The same chemical sensibility determines 

 the aggregation of leucocytes around bacteria or dead tissue, which 

 forms the essential feature of the process of inflammation in higher 

 animals. 



When the reaction of distant parts of the body to a change occurring 

 in any one part depends on the diffusion of some substance from a 

 stimulated part, the total reaction must require a considerable time 

 for its full development. A much more effective method of correla- 

 tion was acquired by the evolution of a nervous system, by means of 



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