DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 173 



branches, particularly in the more basal parts of the system, under natural 

 conditions. Experiment throws light on the character of these transforma- 

 tions. Certain highly susceptible hydroids, e.g., BougainvilliasindPlumu- 

 laria, when kept in standing water in the laboratory, lose their hydranths 

 in a few days, sometimes in a few hours— the fully developed hydranths 

 usually by disintegration, the buds apparently by regression and resorp- 

 tion. A few days later stolons develop in place of hydranths, even from 

 the apical ends of axes, the whole system often showing nothing but 



Fig. 61. — Gonothyraea clarkii. Stolon development from hydranth-stem axes in standing 

 water (from Child, 1923a). 



stolons. Still later, under the same conditions, appearance of hydranth 

 buds suggests some degree of conditioning. With low concentrations of 

 inhibiting' agents these transformations may be even more rapid. In 

 ethyl urethane m/200 Bougaimillia may undergo complete transforma- 

 tion in 48 hours (Fig. 60, A). In KCN m/50,000 the stolons are often 

 subapical, and after a week or more the appearance of hydrant hbuds in- 

 dicates some degree of conditioning, but the buds do not develop beyond 

 an early stage (Fig. 60, B) . A stolon system developing in cyanide from 

 a single apical end is shown in Figure 60, C. Transformation in Gono- 

 thyraea is slower but may take place in standing water (Fig. 61) ; in Obelia 



