316 HEREDITY AND ENVIRONMENT 



means all that lies outside the individual, if it is 

 denned from the standpoint of the entire or- 

 ganism ; but from the standpoint of an organ or 

 cell it is the surrounding organs, cells or fluids 

 of the body — the latter may be denned as "in- 

 ternal environment." If developmental stimuli 

 arise outside the organism they are plainly ex- 

 trinsic or environmental, but if they arise 

 within the organism they are said to be intrin- 

 sic though they may be due to changes in the 

 "internal environment." 



Stimuli are chiefly energy changes of a 

 physical or chemical nature. A stimulus to 

 which an adult organism responds by move- 

 ments or other activities may call forth or in- 

 hibit developmental responses when applied to 

 germ cells or embryos. 



These developmental stimuli may be classed 

 as: 



1. Physical stimuli including the following, 

 (a) mechanical, (b) thermal, (c) electrical, 

 (d) radiant, (e) light, (f ) density of medium, 

 (g) gravity and centrifugal force, etc. 



2. Chemical stimuli include the action of 

 (a) substances found in normal development, 



