PHYSIOLOGICAL GRADIENTS 103 



from the controlling or dominating action of the high 

 end of the gradient. Such physiological isolation is a 

 familiar fact in plants and I have shown that it also 

 occurs in animals and that it determines at least many 

 processes of agamic reproduction. 1 Such physiological 

 isolation may occur in four ways: first, by increase in 

 the size of the body beyond the limit of the axial gradi- 

 ent, the region most distant from the high end of 

 the gradient may be isolated; second, decrease in 

 the activity of the dominant region may decrease the 

 length of the gradient and so determine isolation of the 

 most distant regions without actual increase in size of 

 the body; third, transmission between the dominant 

 region and another part may be blocked by one means 

 or another and parts beyond the block may thus be 

 isolated; fourth, a subordinate part may be directly 

 excited to such a degree that its own activity and 

 electric potential equal that of the dominant part and 

 the current produced by it compensates the current 

 produced by the dominant region, consequently the 

 part concerned is physiologically isolated. 



The result of physiological isolation of a part in the 

 simpler organisms is the same as the result of physical 

 isolation. In all forms in which dedifferentiation 



1 The literature of both botany and zoology contains many data 

 on physiological isolation of parts, but they have been described under 

 various other terms. The outgrowth in plants, in consequence of 

 inhibition of the growing tip, of buds previously inhibited by the tip is 

 a familiar example of physiological isolation. Similar processes occur 

 in the hydroids and other colonial animals. The occurrence of fission 

 and budding and various other forms of agamic reproduction in many 

 different animals also results from physiological isolation. See Child 

 (igoyc, 19100, 1911^, e, 1915^, chap, v, 1917^, d), Child and Bellamy 

 (1919), Bellamy (1919), Hyman (1916). 



