FACTS AND FACTORS OF DEVELOPMENT 85 



operation depends not merely upon the sub- 

 stances used, nor merely upon the way in which 

 the apparatus is set up, but upon both of these 

 things, as well as upon the environmental con- 

 ditions represented by temperature, pressure, 

 moisture or other extrinsic factors. 



4. Heredity and Environment. Unques- 

 tionably the factors or causes of development 

 are to be found not merely in the germ but 

 also in the environment, not only in intrinsic 

 but also in extrinsic forces; but it is equally 

 certain that the directing and guiding factors 

 of development are in the main intrinsic, and 

 are present in the organization of the germ 

 cells, while the environmental factors exercise 

 chiefly a stimulating, inhibiting or modifying 

 influence on development. In the same dish 

 and under similar environmental conditions, 

 one egg will develop into a worm, another into 

 a sea urchin, another into a fish, and it is cer- 

 tain that the different fate of each egg is de- 

 termined by conditions intrinsic in the egg 

 itself, rather than by environmental conditions. 

 We should look upon the germ as a living 

 thing, and upon development as one of its 



