514 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



The second essential condition for the maintenance of life of the 

 cell aggregate is the coordination of its parts and the due regulation 

 of their activity, so that they ma}'' work together for the benefit of 

 the whole. In the animal body this is ejffected in two ways: First, 

 through the nervous system; and, second, by the action of specific 

 chemical substances which are formed in certain organs and carried 

 by the blood to other parts of the body, the cells of which they excite 

 to activit3^ These substances have received the general designation 

 of "hormones" (bpfida), to stir up), a term introduced by Prof. 

 Starling. Their action, and indeed their very existence, has only 

 been recognized of late years, although the part which they play in 

 the physiology of animals appears to be only second in importance 

 to that of the nervous system itself; indeed, maintenance of life may 

 become impossible in the absence of certain of these hormones. 



NERVOUS SYSTEM IN MAINTENANCE OF AGGREGATE LIFE. 



Before we consider the manner in which the nervous system serves 

 to coordinate the life of the cell aggregate, let us see how it has 

 become evolved. 



The first step in the process was taken when certain of the cells of 

 the external layer became specially sensitive to stimuli from outside, 

 whether caused by mechanical impressions (tactile and auditory 

 stimuli) or impressions of light and darkness (visual stimuli) or 

 chemical impressions. The effects of such impressions were prob- 

 ably at first simply communicated to adjacent cells and spread from 

 cell to cell throughout the mass. An advance was made when the 

 more impressionable cells threw out branching feelers amongst the 

 other cells of the organism. Such feelers would convey the effects of 

 stimuli with greater rapidity and directness to distant parts. They 

 may at first have been retractile, in this respect resembling the long 

 pseudopodia of certain Rhizopoda. When they became fixed they 

 would be potential nerve fibers and would represent the beginning of 

 a nervous system. Even yet (as Ross Harrison has shown), in the 

 course of development of nerve fibers, each fiber makes its a,ppear- 

 ance as an amoeboid cell process which is at first retractile, but 

 gradually grows into the position it is eventually to occupy and in 

 which it will become fixed. 



In the further course of evolution a certain number of these 

 specialized cells of the external layer sank below the general surface, 

 partly perhaps for protection, partly for better nutrition, they 

 became nerve cells. They remained connected with the surface by 

 a prolongation which became an afferent or sensory nerve fiber, and 

 through its termination between the cells of the general surface con- 

 tinued to receive the effects of external impressions; on the other 



