336 HEREDITY AND ENVIRONMENT 



parts are kept active while other parts are 

 kept inactive or are inhibited. 



Normal development is dependent upon the 

 correlated activity of many parts of the or- 

 ganism. If in any part stimuli and responses 

 are lacking the development of that part is 

 arrested or inhibited. For example in the 

 cleavage stages different kinds of plasm are 

 sorted and localized by protoplasmic move- 

 ments within cells and these substances are 

 then isolated by cell divisions and by the for- 

 mation of partition walls between cells; these 

 protoplasmic movements occur in response to 

 stimuli and if these movements are stopped 

 cleavage and differentiation are arrested. In 

 later stages the infolding of the gastrula, 

 or neural tube, or alimentary canal, and 

 the folding of layers in general, which plays 

 so important a part in development, are 

 due to the movements of substances within 

 cells and to the movements of cells in the 

 layers in which they lie, and if these move- 

 ments are inhibited normal development is 

 prevented. 



