THE RANGE OF DOMINANCE 129 



by accumulations of red pigment. Various facts, some 

 of which have been mentioned above (pp. 79, 96-99), 

 show that the parts of the hydranth are determined 

 from the apical end in the basal direction. The point 

 of present interest in this process is the length of stem 

 concerned in the formation of the new hydranth and 

 the length of each of the four distinguishable regions, 

 a, b, c, d, of the developing hydranth. In pieces of like 

 physiological condition kept under the same external 

 environment these lengths show a high degree of con- 

 stancy, but they can readily be altered by altering the 

 metabolic rate in the pieces. Fig. 60 shows the length 

 and proportions of the early stage of a hydranth develop- 

 ing with a medium metabolic rate. Fig. 61, with a high 

 rate, and Fig. 62, with a very low rate. Evidently the 

 higher the metaboHc rate the greater the distance from 

 the end of the stem and from each other at which the 

 two rows of tentacles arise. The relative lengths of 

 the different parts also change with metabolic rate, 

 that of the region a increasing and that of the region 

 d decreasing with increasing metabolic rate, and vice 

 versa. ^ 



The position, size, and time of appearance of hy- 

 dranths and the relation of hydranths to other parts in 

 the reconstitution of isolated pieces of Tubularia and 

 related forms have been repeatedly investigated, but, 

 although the facts are very definite, the various authors 



^ Child, "An Analysis of Form Regulation in Tubularia. II, 

 Differences in Proportion in the Primordia," Archiv fur Entwickelungs- 

 mechanik, XXIII, 1907. In this paper I showed that such differences in 

 proportion appeared in hydranths from different levels and ends of the 

 stem, but it is now known that these differences in level really represent 

 differences in metabolic rate. 



