ORIGIN OF AXIATE PATTERN 53 



but as a center of differentiation in which the intensity 

 of the process diminishes as the distance from the center 

 increases, until it passes away into an indifferent region. 

 Many other systems, such as the nose, ear, hypophysis, 

 gills, seem to have the same indefinite boundaries which 

 may even overlap one another" (Harrison, 1918, p. 456). 

 Evidently Harrison conceives these primordia as gradi- 

 ents in activity in a more or less specialized cellular 

 region of the embryo. Such gradients differ from the 

 general axial gradients of the body only in that they 

 are determined in some way, presumably by intra- 

 organismic correlative conditions in specialized body 

 regions, and are concerned with particular organ com- 

 plexes instead of with the body as a whole. In still 

 other cases new polarities are apparently determined 

 and localized by slight differences in activity between 

 different cells of a mass. Such differences determine the 

 more or less definite localization of a region of growth in 

 which the activity decreases toward the periphery, and 

 as growth progresses an axial gradient arises. Determi- 

 nation of new polarities in this way apparently occurs 

 in many cases when pieces of naked hydroid stems give 

 rise to multiple stolons, each of which represents a new 

 axis and a new gradient. These multiple polarities 

 have been observed by many investigators, and I have 

 been able to produce them experimentally in hydrozoan 

 planulae by first obliterating the original polarity through 

 differential inhibition. In the origin of adventitious 

 buds from the epidermal cells of the Begonia leaf similar 

 local growth areas with gradients in activity from center 

 toward periphery and from the surface inward are the 

 first indications of the new plant axes (Regel, 1876; 



