124 C. M. CHILD. 



agreement with other data on both animals and plants and con- 

 stitute another bit of evidence in support of the conclusion that 

 physiological axes are primarily gradients in rate of fundamental 

 dynamic processes and in associated physical and structural 

 conditions in living protoplasm. 



The most important facts are summarized as follows: 



1. In the hydranths of Pennaria, Tubularia, BoiigainviUea, 

 Obelia, Gonotliyria and various other forms, the susceptibility 

 to lethal concentrations of at least many external agents is great- 

 est in the apical region of each tentacle and of the hydranth 

 body and decreases basipetally. 



2. In the elongated medusa bud of Pennaria the suscepti- 

 bility decreases basipetally from the apical region. 



3. Stems are in general much less susceptible than the more 

 highly specialized structures, though the perisarc may act as a 

 protective covering to some degree. At least in the more apical 

 regions of axes the susceptibility of the stem also decreases basi- 

 petally, but in the more basal regions irregularities may appear. 



4. Physiologically older zooids or medusa buds are less sus- 

 ceptible than younger, except in certain cases where the develop- 

 ment of motility increases susceptibility to such an extent as to 

 overbalance the age differences. 



5. Except in the earlier stages the medusa buds of Pennaria 

 are less susceptible to lethal concentrations than the hydranth 

 bodies from which they arise, and the gonophores and gonozooids 

 of other forms show the same relations. 



6. In the colony as a whole and in each axial complex the 

 average susceptibility of well developed hydranths decreases 

 basipetally, but certain characteristic differences in axial rela- 

 tions exist in the monopodial (Pennaria) and the sympodial 

 type (Obelia). 



7. The medusa buds of Pennaria and Bongainmllea and the 

 gonozooids of Obelia, etc., appear first on those hydranths or in 

 those regions of the colony where the susceptibility to lethal con- 

 centrations of agents is least, i.e., in the basal regions of the more 

 basal branches. From these regions the zone of development of 

 these parts gradually progresses apically as the different levels 

 successively attain the physiological condition which determines 

 their development. 



