III. 8 CHEMICAL COMPOSITION 155 



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9. SUMMARY 



In the numerous experiments which we have been considering 

 the effect is observed upon the development of the embryo of 

 certain alterations in the constitution of that embryo's normal 

 environment. Either some factor which is not usually present 

 is added to the environment, or else some factor which is 

 customarily found there is altered by increase or decrease, or 

 removed altogether. 



In some cases development remains undisturbed by this treat- 

 ment, in others it may be merely generally retarded or accelerated, 

 in others again it may be altered not merely in rate but in form, 

 with the production of an abnormality or monstrosity, and if its 

 effect is too intolerable the death of the embryo may ensue. 



Throwing light as they do on the causes of the formation of 

 natural monsters, such experiments are no doubt of the highest 

 interest from a general teratological point of view. The mere 

 possibility of the occurrence of such malformations is, however, 

 itself a fact of the deepest morphological significance. A monster 

 is an organism in which the development of some part or parts 

 has either exceeded or fallen short of its normal limitations, and 

 any such phenomenon points indubitably to a certain mutual 



