STRUCTURE AND DEVELOPMENTAL RATE 255 



be supposed that every mammal would develop some form of can- 

 cerous growth should it chance to live until extreme old age. 

 The increased length of life in man may be associated with the 

 increased frequency of cancer. 



13. GENERAL SUMMARY 



In considering the results of the present study it is necessary to 

 recognize the fact that a given animal species passes through its 

 embryonic stages at a specific rate of development, probably 

 dependent upon the rate of oxidation in the protoplasm of the 

 species. This developmental rate varies within certain normal 

 limits; should variations in rate extend beyond these limits, the 

 developmental result frequently becomes modified and distorted. 



The rate of development is not uniform throughout the entire 

 process, but periods of rapid progress alternate with moments 

 of slow rate or almost quiescence. In spite of these changes in 

 rate, development in most forms does not actually stop after it 

 has once started, but progresses in a continuous manner until the 

 fully formed animal is produced. 



There are certain animals in which the continuous mode of 

 development has become modified. In these forms development 

 begins and attains a definite stage and then stops completely, to 

 remain at a standstill for days or even weeks, until a change in 

 the environment again permits the resumption of the develop- 

 mental processes and the completion of the fully formed animal. 

 Such a discontinuous mode of development is universal among 

 the birds and is known to occur in several mammals. 



With these points in mind, the results of the present study may 

 be summarized as follows: 



1 . The continuous mode of development may be experimentally 

 changed into the discontinuous by two very simple methods, tem- 

 porarily lowering the surrounding temperature and thereby re- 

 ducing the rate of oxidation and by directly cutting off the sup- 

 ply of oxygen. 



The effects on subsequent development of interruptions caused 

 by these methods depends upon the stage during which the inter- 

 ruption is introduced. There are stages of apparent indifference 

 to a stop in development. Shortly after gastrulation is completed, 



