EARLY DEVELOPMENT IN THE FROG. 34" 



certain conclusions. These are briefly: (i) that differences in 

 susceptibility must depend primarily upon quantitative dif- 

 ferences in protoplasmic condition, rather than upon the specific 

 or qualitative differences, and (2), that these quantitative dif- 

 ferences are closely associated with differences in the rate at 

 which fundamental physico-chemical processes are taking place 

 in different regions of an individual or in different indivduals. 



These facts, viz., differential susceptibility relations and their 

 marked uniformity and lack of specificity, not only in the frog 

 but also in other 'organisms; gradients in protoplasm-yolk and 

 pigment distribution both of which parallel the polar axis; and 

 the fact that development begins and proceeds in a definite, 

 orderly, and sequential manner with respect to certain axes (or 

 to the polar axis) in such a way that structural and functional 

 order are at all times referable to one or more axes or planes of 

 symmetry (anterior-posterior, dorso-ventral, medio-lateral), con- 

 stitute evidence for the existence of a fundamental underlying 

 order of some sort that exists prior to, and which conditions the 

 orderly, sequential, and spatial relations of parts that become 

 apparent later in development. Furthermore it is this order, of 

 which differential susceptibility is one expression, which affords 

 the basis for the great uniformity and lack of specificity in the 

 susceptibility relations, not only in the frog but also in other 

 organisms. 



The nature and origin of this underlying order is the problem 

 of physiological axiation a problem that has been the subject 

 of no little speculation and experimentation. It has been thought 

 by some that polarity and symmetry as manifested in the organ- 

 ism are fundamental properties of protoplasm, and, like the 

 polarity and symmetry of crystalline substances or of the magnet, 

 are intra-molecular in character. It may be said, however, 

 that this conception and others of a similar nature, do not stand 

 the test of experiment even where capable of such an analysis. 



On the basis of extensive experimental work Child has arrived 

 at a dynamic conception of physiological axes in the organism 

 with the aid of which he accounts in a satisfactory manner not 

 only for many of the hitherto unrelated facts of normal and 

 abnormal development, but also he has been able experimentally 



