; Influence of Environment 209 



movements or other activities may call forth or inhibit develop- 

 mental 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, such as oxygen, carbonic acid, water, 

 food, secretions of ductless glands, etc. and (b) substances not 

 found in normal development, such as various salts, acids, alkalis, 

 alcohol, ether, tobacco, etc. 



3. General vs. Specific Stimuli. In general the action of these 

 stimuli during development does not call forth a perfectly specific 

 and definite response of the organism; various stimuli may pro- 

 duce the same result. Thus artificial parthenogenesis has been 

 produced by almost every stimulus named, and weakened or re- 

 tarded development is produced by many different stimuli. 



By the elimination of certain of these stimuli which are normally 

 present or by introducing stimuli which are not usually present 

 very important and even profound changes in development may 

 be produced. . In this way animals have been formed which are 

 turned inside out, or side for side, or in which heads or nervous 

 systems or muscles or backbones are lacking, or in which the 

 various organs are not found in normal positions. In this way 

 dwarfs and giants and one-eyed monsters as well as all sorts 

 of double and partial embryos have been formed. In general 

 monstrous and defective forms of development are due to altera- 

 tions of the normal environment rather than to defective heredity. 



II. DEVELOPMENTAL RESPONSES 



The character of developmental responses to stimuli depends 

 primarily upon (a) the nature of the organism and (b) the stage 

 of development at which the stimulus acts. Modifications are 

 more easily produced and are more profound during cell division 

 than during intervening periods and at early stages of develop- 



