THE AXIAL GRADIENTS IN HYDROZOA. 213 



wide observation by both of us, and agrees well with the other 

 data on susceptibility. In a later paper it will be shown that 

 similar relations exist in other hydrozoa. 



THE SUSCEPTIBILITY OF SEXUAL ANIMALS. 



A few observations have been made on gonad-bearing animals 

 with ether as agent. In all cases observed, the gonads are less 

 susceptible than other regions of the animal. As in the case 

 of early buds, the low susceptibility of the gonads is perhaps not a 

 correct criterion of their physiological condition as compared with 

 other parts of the body, for the reproductive cells are of course 

 entirely separate from the muscular mechanism and are probably 

 not very greatly affected by excitation and contraction in other 

 cells. The number of sexual individuals available has been too 

 small to permit definite conclusions concerning the comparative 

 susceptibility of sexual and non-sexual animals, though the few 

 data at hand indicate that sexual animals are in general less sus- 

 ceptible than non-sexual. 



SUSCEPTIBILITY IN RELATION TO CONCENTRATION. 



Since the present paper is primarily concerned with other prob- 

 lems than those connected with concentration of agents, the data 

 on concentration are incidental and presented merely to show 

 approximately how rapidly the processes of disintegration occur 

 in concentrations most commonly used. There is of course 

 some individual variation in susceptibility, doubtless dependent 

 on uncontrolled factors such as nutritive condition, physiological 

 age, motor activity, etc., and the course of disintegration differs 

 under different conditions, as already described. A few char- 

 acteristic data for large, well fed, asexual individuals of H. oli- 

 gactis at temperature 2o-22 C. are tabulated for illustration 

 and comparison. 



The concentrations given in the table for KNC, alcohol, and 

 ether are of course only approximate. All solutions of these 

 agents were freshly made in every case and loss from volatiliza- 

 tion was prevented as far as possible. As regards methylene 

 blue and Janus green, the tabulated data are all from solutions 

 of the same sample of each. Different preparations of these 



