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 unicellar 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 con- 

 sequence 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 organ- 

 isms 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 collection 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 develop- 

 ment. A much more effective method of correlation was acquired by the evo- 

 lution of a nervous system, by means of which the consensus partium could be 

 maintained by the rapid propagation of molecular changes along differen- 

 tiated paths in the protoplasm. The development of this second mode of 



1178 



