DOMINANCE AND PHYSIOLOGICAL ISOLATION 315 



tained, the stolon tip alters its reaction, turns away from the substratum, 

 and its tip becomes a new hydranth (Fig. 108, B). In relation to this 

 hydranth a new stem develops, and later one or more new stolons grow 

 out from its base, their tips becoming hydranths when suflliciently isolated 

 physiologically from the dominant hydranth. In some species successive 

 stems develop in this way at almost equal distances in series; in others 

 the sequence is less regular. With still further increase in length of the 

 original stem hydranth buds may arise in some species along the stolon 

 and later from the proximal region of the stem itself. Each of these de- 

 velops a stem and becomes a branch. Buds may also appear if the apical 

 hydranth is removed and kept from reconstitution ; but when it is present 

 and in good condition, new hydranth development begins only at a certain 

 distance from it. From the distal end of a stem piece several centimeters 

 long a hydranth develops rapidly, and usually a second smaller hydranth 

 develops more slowly from its proximal end (Fig. 108, D). Sometimes, 

 however, particularly in certain species, a stolon appears at the proximal 

 end (Fig. 108, C), and its tip transforms later into a hydranth. Stolons 

 develop more frequently from the more proximal levels of the stem (Child, 

 1907a) . In various hydroids apical regions can be transformed into stolons 

 by inhibiting conditions (pp. 172-75) : in Tubularia the stolon is evidently 

 an axis somewhat inhibited by the dominant hydranth or by other con- 

 ditions. It develops as a bud from the lower levels of the stem gradient 

 in consequence of partial physiological isolation, and with increased isola- 

 tion its tip becomes a hydranth. When the hydranth, the region of chief 

 dominance, is removed, the degree of isolation at the proximal end of a 

 piece may be sufhcient to permit development of a hydranth there at 

 once, or a short stolon may develop first (Fig. 108, C, D). So-called "axial 

 heteromorphosis" in Tubularia differs from the natural agamic reproduc- 

 tion only in that removal of the dominant hydranth permits a sufficient 

 degree of isolation for hydranth development at a shorter distance from 

 the distal end than in the intact animal. In pieces the rate of hydranth 

 development decreases from distal to proximal levels, except in very long 

 pieces, in which the proximal end is already more or less physiologically 

 isolated, and a rapidly developing oral hydranth retards still further the 

 development of the aboral hydranth. This is shown by the more rapid 

 development of the aboral hydranth when development of the oral hy- 

 dranth is retarded or inhibited and also by the more rapid development of 

 a hydranth at the distal end of a proximal piece than at the proximal end 

 of the piece immediately distal to it. If oral and aboral hydranth develop- 



