Vol. XLV. October, 1923. No. 4. 



BIOLOGICAL BULLETIN 



THE AXIAL GRADIENTS IN HYDROZOA. V. EXPER- 

 IMENTAL AXIAL TRANSFORMATIONS IN 



HYDROIDS. 



C. M. CHILD, i 



The hydroid stolon usually appears in nature as a basal out- 

 growth readily distinguishable from the stem by its habit of 

 growing in contact with surfaces rather than free in the water. 

 Contact has often been regarded as a factor in determining its 

 formation, but it has been observed by many investigators 

 that in the reconstitution of isolated pieces of hydroid stems 

 stolons may develop from that end of the piece which was origi- 

 nally apical, as well as from the basal end, even when these ends 

 are not in contact with solid surfaces. In some cases also it has 

 been observed that isolated pieces of certain species develop 

 only stolons which may later give rise to hydranths or may con- 

 tinue to grow as stolons. Peebles (1900) states that this fre- 

 quently occurs in pieces of Hydractinea and Podocoryne when 

 they are left in dishes undisturbed without change of water. 

 Loeb ('92) maintained that stolon formation in Antennularia 

 antennina is determined by gravity, but Morgan ('01) and 

 Stevens ('02, '10), while not denying the correctness of Loeb's 

 conclusions, demonstrated beyond a doubt that other factors 

 than gravity may be concerned. They were not able, however, 

 to reach definite conclusions concerning the nature of these 

 factors. Lund ('21) has shown that when isolated stem pieces 

 of certain hydroid species are exposed to the electric current, 

 stolons tend to arise at the end toward the cathode, hydranths 

 at the end toward the anode. Various other data might be cited , 

 but it seems fair to say that no one thus far has been able to 

 discover a general physiological basis for the development of 



1 From the Hull Zoological Laboratory, University of Chicago. 



181 



