THE AXIAL GRADIENTS IN HYDROZOA. 



I. HYDRA. 



C. M. CHILD AND L. H. HYMAN, 

 DEPARTMENT OF ZOOLOGY, UNIVERSITY OF CHICAGO. 



In the development and formulation of the general conception 

 of the axial gradient conditions observed or experimentally in- 

 duced in Tubularia and other Hydrozoa have played a part 

 (Child, 'i$c, pp. 79, 91-2, 96-99, 128-137), but particularly dur- 

 ing the last five years data concerning the gradient in these forms 

 have been accumulating until now a point has been reached where 

 the volume and variety of these data warrant their presentation. 



While some of these data have been briefly considered, as 

 noted above and in other papers, most of them have not as yet 

 been recorded, and it is proposed to bring them together in con- 

 nected form in this and two or three following papers. In the 

 present paper the data on three species of Hydra are presented. 



METHODS AND MATERIAL. 



The gradients in Hydra have been studied chiefly by means of 

 the direct or lethal susceptibility method 1 with cyanide in con- 

 centrations ranging from m/i,ooo to m/io, ethyl alcohol 1-5 per 

 cent., ethyl ether 1-3 per cent, and the dyes, neutral red, methyl- 

 ene blue and Janus green in wide ranges of concentration. With 

 these agents the regional differences in susceptibility were de- 

 termined, and the course of the gradient of disintegration and 

 death in body and tentacles was observed under various condi- 

 tions. Potassium permanganate, which has been found useful 

 in many cases as a means of demonstrating the gradients as color 

 gradients, has also been used to some extent (Child, '19), but is 

 rather unsatisfactory because of the extreme contraction pro- 

 duced. 



Observations have been made on the three common species of 



1 Child, '130, b, '146, '150, b, chap. III., '15^, 'i6a, b, c, '17, Hyman, "16, 17. 



183 



