THE MUTATED GENE 47 



the whole set of recent experimentation upon amphibia might be 

 quoted. But what is important for our present discussion is the 

 production of monsters identical with those found in nature and of 

 essentially the same type as those caused by mutant genes. 

 We shall therefore not report upon experiments of transplantation 

 or removal of embryonic parts, which lead to monstrosities, 

 as these are not comparable to the effect of gene action, though 

 analyzing directly the localization of decisive materials (Lewis, 

 Spemann, Harrison, Holtfreter, etc.). Stockard (1913) was 

 the first to produce typical monsters, especially cyclopia, by the 

 action of magnesium salts upon early embryonic stages of 

 Fundulus. Other authors as well as Stockard himself found later 

 that the same effect could be brought about by many agencies 

 such as anesthetics (McClendon), acetone and butyric acid 

 (Werber, 1916), low temperature (Kellicott, 1916; Stockard, 

 1913), ultraviolet rays (Hinrichs, 1925), and X-rays (Little and 

 Bagg, 1929). This reminds us of the early experiments with the 

 butterfly wing, where about the same agencies proved effective 

 in changing the pattern. And the comparison is still closer if 

 we add that in the experiments just reported there is also a 

 sensitive period, viz., from the fertilized egg up to the stage of 

 neurula. It is therefore not surprising to see that Stockard 

 (1921) derived the same explanation, viz., an action of all these 

 factors upon the speed of certain developmental processes. 

 He states that all methods employed have more or less the same 

 effect, without apparent specificity. (Notice the parallelism with 

 the phenocopies.) The reason is that all these methods primarily 

 slow up the differentiation of the embryo. The state at which 

 this happens decides which type of monster will be produced. 

 Not all stages of development react, however, in the same way; 

 e.g., the treatment of fish eggs before gastrulation produces mostly 

 double monsters; after gastrulation, abnormalities of the eye. 

 Other periods of development seem to be inaccessible to such 

 experiments. From this it follows that there are definite sensitive 

 periods for this type of treatment, and they are found near the 

 period of visible origin of the organ in question. Stockard 

 thinks that the actual basis of the phenomenon of sensitive 1 

 periods is to be found in the assumption that the organ at this 

 time grows and differentiates faster than the others. A retarda- 

 tion at this moment, therefore, has a differential effect. If we 



