PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 165 



kind of living. They appear to be the primary factors in bringing about 

 axial and regional differences in rate of development and in form and 

 proportions. 



Eggs of different animal species differ greatly at the beginning of em- 

 bryonic development : in some there is apparently little or nothing more 

 in the way of developmental pattern than differentials in rate ; others give 

 direct or indirect evidence of more or less specific regional differentiation 

 in the cytoplasm. Even in these, however, the cytoplasmic differentia- 

 tions apparently are, or become, localized in definite relation to a general 

 gradient pattern and are probably to be regarded, like other features of 

 development, as expressions or consequences of that pattern, appearing 

 in the egg rather than in later stages (see chap. xiv). Gradient pattern 

 along any physiological axis is itself a three-dimensional pattern, a gradi- 

 ent system; it may perhaps be regarded as a general background on which 

 details are gradually filled in, or as a frame of reference within which, 

 and in relation to which, developmental events occur. But unless the 

 following pages are wholly in error, it is more than a background or a 

 frame of reference; it is a physiologically active and effective factor in 

 initiating the order and unity of the individual. 



