34 LIBBIE H. HYMAN. 



organism as a whole and to its parts. Consequently secondary 

 gradients are of common occurrence in complex animals. 



Owing to the fact that the disintegration gradients are observ- 

 able and identical in a wide range o*f substances of very varied 

 chemical properties and constitutions, it is certain that they are 

 not due to any specific action of these substances upon proto- 

 plasm. This is further evidenced by the fact that organisms dis- 

 play the same differential susceptibilities to extremes of physical 

 conditions, such as high temperatures, and to low oxygen supply. 

 Owing to the fact that differential susceptibility is directly re- 

 lated to physiological conditions such as age, starvation, regenera- 

 tion, motor activity, stimulation, etc., it is further certain that 

 differential susceptibility is not primarily an expression of struc- 

 tural or morphological gradations along the axes of organisms. 

 All of the facts at hand lead us irresistibly to the conclusion that 

 the gradients are physiological in nature, that they are mani- 

 festations of a quantitative gradation in function and metabolism 

 along the axes of organisms. We are therefore accustomed to 

 refer to these gradients as metabolic gradients ; physiological 

 gradients would possibly be a better term, as Professor Child has 

 recently suggested. 1 



The susceptibiltiy method is thus a method for determining in 

 a general way the relative rates of activity of different parts of 

 the organism. Its results do not necessarily correspond to de- 

 terminations of total metabolism since usually the susceptibility 

 of only certain parts of the organism can be determined. It is 

 not claimed that it is an accurate measure of metabolic rate, since 

 too many factors enter into differential susceptibility, or that it 

 should supplant methods of directly measuring metabolic rate, 

 such as oxygen consumption, carbon-dioxide production, or de- 

 termination of other metabolic end products. It has, however, 

 served to reveal facts not discoverable by any other method at 

 present known to us. 



i These matters are discussed at greater length in a paper by Child now 

 in press in the BIOLOGICAL BULLETIN and in a book " The Origin and Devel- 

 opment of the Nervous System," shortly to be published by the University of 

 Chicago Press. I have had the privilege of reading the manuscript of both 

 publications and wish to acknowledge my indebtedness to them for the present 

 argument. 



