308 HEREDITY AND ENVIRONMENT 



means all that lies outside the individual, if it is 

 defined 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 defined 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, 



