90 THE CONTROL OF LIFE 



While some developing organisms are callously in- 

 different to changes in the environment, there are others 

 which respond sensitively, sometimes in a startling way, 

 to external changes, even when they do not seem to be 

 very drastic. Thus Professor Jacques Loeb has shown 

 (Biolog. Bull., XXIX, 1915, p. 50) that it is very easy 

 to produce a percentage of minnow-embryos (Fundulus) 

 with defective eyes, by the simple expedient of adding a 

 minute quantity of potassium cyanide to the water, 

 or by exposing the newly fertilised eggs to low tempera- 

 ture. That is to say, relatively slight environmental 

 changes may set a-going vital processes which alter the 

 constitution of the developing embryo in a very precise 

 way. A leap is taken in the direction of blindness. 



For the same fish (Fundulus), it has been shown by 

 Stockard (Journ. Exper. Zool, Feb., 1909) that the 

 addition of a very minute quantity of magnesium salt 

 to the water induces in a large number of embryos the 

 development of a single Cyclopean eye instead of the 

 normal two eyes. 



We must also recall the experiments of Professor D. T. 

 MacDougal, who injected solutions of sugar and salts 

 of calcium, potassium, and zinc into the developing 

 ovaries of one of the Evening Primroses with the result 

 that a small percentage of the seeds developed into 

 notably atypical plants, which bred true to the third 

 generation. The chemical substances introduced were 

 not of a very out-of-the-way sort ; they were not very 

 different from those which might occur in the course of 

 nature in the sap of the plant. Among the changes 



