THE PHYSIOLOGY OF DEVELOPMENT 203 



The first postulate of Child's hypothesis which need 

 here concern us may be stated thus : — the morphological 

 differentiation of parts (axiate pattern) with reference to a given 

 axis is preceded by the appearance of a gradient of physio- 

 logical activity (axial gradient) along this axis. This proposi- 

 tion may be shown to be true in a number of ways ; and is 

 quite independent of the particular interpretation of the axial 

 gradient which may be stated later. The existence of a ph3^sio- 

 logical gradient is here taken to imply that there exist between 

 the properties of the cells quantitative differences following a 

 definite orientation with reference to the future axiate pattern. 

 A very clear instance of this is provided by experiments on 

 asexual reproduction in Planaria dorotocephala. If a large 

 number of individuals of this species are cut up into strips, 

 it is found that the frequency with which corresponding strips 

 taken from different regions regenerate a head and develop into 

 complete new individuals varies in a perfectly definite way. If 

 we plot the statistical results of such an experiment with fre- 

 quency of head-formation as ordinates and regional position of 

 the strip along the abscissa (taking the head extremity as zero), 

 the ordinates gradually diminish as we pass along the x-axis 

 up to a certain point, then increase abruptly to a new maximum 

 and then diminish (Child, 19 15). Thus before there exists 

 any outward structural appearance of the formation of a new 

 head, there exists a physiological difference in the tissue at 

 the point where the new head is to be formed. 



The second proposition brings us on to more debateable 

 ground. To do justice to the author, it may be stated, in his 

 own words : " Axial gradients have often been called metabolic 

 gradients, because differences in metabolism, or more 

 specifically of oxidative metabolism as indicated by various 

 experimental methods, appear to be characteristic and con- 

 spicuous features of them." In this sentence for the first 

 time an attempt is made to put the problem of individuation 

 on a basis which is accessible to quantitative methods of 

 physiological inquiry. 



Various methods have been employed by Child and his 

 co-workers in the attempt to demonstrate regional differences 



