328 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; these substances are iso- 

 lated by cell divisions and by the formation of 

 partition walls between cells, which are 

 brought about by protoplasmic movements in 

 response to stimuli; if these movements are 

 stopped cleavage and differentiation are ar- 

 rested. In later stages the infolding of the 

 gastrula, the neural tube, the 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 be inhibited normal development is 

 prevented. 



Another type of fimctional activity which is 



