172 C. M. CHILD. 



In the actinian Harenactis the localization of a region of more 

 rapid and more extensive growth by injury is sufficient under 

 certain conditions to determine the position and development of 

 new polarities (Child, '09, 'iob, 'i$c, Figs. 79-83). ' The axial 

 gradation results in this case from the fact that the activity of 

 the cells is greatest in the middle region of such an area and de- 

 creases toward its borders. In this connection a recent statement 

 of Harrison's concerning the limb-rudiment of Amblystoma is 

 of interest. Harrison says : " The limb rudiment may be thus 

 regarded, not as a definite circumscribed area like a stone in a 

 mosaic, but as a center of differentiation in which the intensity 

 of the process diminishes as the distance from the center in- 

 creases, 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, '18, p. 456). In other words, Harrison 

 conceives these primordia as gradients 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 intraorganismic 

 correlative conditions in specialized body regions, and are con- 

 cerned with particular organ complexes 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 axis arises. Determination of new polarities in this way ap- 

 parently 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 planulse by first ob- 

 literating 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, '76, Child, 



