350 A. W. BELLAMY. 



shown that this distance decreases with increasing inhibition 

 (decrease in metabolic rate) of the apical region, and increases 

 slightly with increase in activity (acceleration) of the apical 

 region. The facts suggest at once a definite physiological correla- 

 tion between the apical region and the region of the dorsal lip 

 which arises secondarily as a posterior region of active growth 

 and differentiation, and which does not appear to differ funda- 

 mentally from the posterior growing region which gives rise to 

 the trunk in annelid larvae (Child, 19170). 



From these facts, viz., the secondary origin of the dorsal lip 

 region, its definite spatial relation to the apical region, which 

 may be controlled experimentally, and its appearance at the 

 distal end of a rapidly growing region, the origin of the dorsal 

 lip region offers an interesting parallel to those cases in certain 

 lower animals and plants where new zooids or new individuals 

 arising by asexual reproduction have been shown to appear as 

 physiological isolations. 1 



In the simple cases of physiological isolation in lower animals 



1 The role of physiological isolation, dominance and subordination in develop- 

 ment has been studied by Child and fully stated by him (191 sa, pp. 88-169) and 

 need not be considered here except to mention some of the pertinent facts. In 

 Child's own words: 



"The experimental evidence demonstrate, first the essential independence of 

 the apical region in both plants and animals, and secondly, determination and 

 control by this apical region of the developmental processes at other levels of the 

 major axis of the individual." (19150, p. 125.) 



The facts indicate for those forms investigated, that the control of subordinate 

 parts by the dominant region is transmissive rather than transportative in character, 

 and that the range of dominance, i.e., the distance over which the control of the 

 dominant region is effective and beyond which physiological isolation occurs, 

 varies with the relative metabolic rates of the dominant and subordinate parts, 

 and condition of the conducting path; increasing with increase in metabolic rate 

 in the dominant regions and with the specialization of the conducting path, and 

 decreasing with decrease in the metabolic rate of the dominant region, with increase 

 in rate of the subordinate region, and with interference with the conducting path. 

 These four ways in which physiological isolation may occur are summed up by 

 Child, thus: 



"It is possible to control and alter experimentally the spatial relations of parts 

 in the individual by altering the length of the metabolic gradient and so the range 

 of dominance. Parts of the individual may come to lie beyond the range of domi- 

 nance in consequence of increase in size of the whole, of decrease in range and degree 

 of dominance by decrease in the metabolic rate in the dominant region, of decrease 

 in conductivity of the paths of correlation, and of the direct local action of external 

 factors which increase the independence of subordinate parts." (19150, p. 169.) 



