i894. ''PREFORMATION OR NEW FORMATIONS 297 



containing it, increases in bulk by growth. Thirdly, the specialisation 

 and dififerentiation of the organs and tissues is due to the action of 

 varied external stimuli upon the same organic substance. In his 

 own words : — 



" My theory may be called evohitionistic, because it assumes the 

 •existence of a specific and highly-organised incipial plasm as the 

 basis of the process of development. It may be called epigenetic, 

 because the incipia grow and elaborate from stage to stage only in the 

 presence of numerous external conditions and stimuli beginning with 

 the metabolic processes preceding the first cleavage of the egg-cell, 

 until the final product of the development is as different from the 

 £rst incipia as adult animals and plants are from their constituent 



cells." 



P. Chalmers Mitchell. 



